Plant Diseases in Thailand
The seven disease groups that matter on Thai farms — what causes each one, what it looks like in sequence, the temperature and humidity numbers that decide whether it happens, and how to tell it apart from a nutrient problem. Part two of three.
On this page
Three labels appear throughout. VERIFIED — from a named published source, listed on the sources page. INFERRED — our reasoning from verified data, usually a measured threshold read against Thai climate normals, not itself measured in Thailand. UNVERIFIED — we looked and could not confirm it.
Root rot: Pythium and Phytophthora
Thai โรครากเน่าโคนเน่า · Vietnamese thối rễ · Oomycota — water moulds
They are not fungi. Pythium and Phytophthora belong to the Oomycota, a lineage closer to diatoms and brown algae than to fungi; their cell walls are cellulose, not chitin VERIFIED. This is not a taxonomic footnote. Oomycetes produce motile, swimming zoospores that travel through free water in the substrate, the drip lines and any recirculating tank VERIFIED. True fungi do not swim. That single fact is why water management and water disinfection dominate the control of this group and are close to irrelevant for powdery mildew.
What is recorded in Thailand
A 2022 Thai study recovered 11 species of Pythium and allied genera from Chanthaburi, Rayong and Trat — P. acanthicum, P. deliense, P. diclinum, P. longipapillum, P. myriotylum, P. torulosum, Globisporangium carolinianum, G. splendens, Phytopythium cucurbitacearum, P. helicoides and P. vexans, including the first Thai record of P. longipapillum VERIFIED. Phytopythium helicoides and P. vexans are warm-adapted root and crown rot pathogens frequently misidentified as Pythium VERIFIED.
Note the honest limit: that survey covered natural and field habitats in eastern Thailand, not greenhouses, and it does not include P. aphanidermatum. We could not find a Thai record of P. aphanidermatum or a consolidated Thai Phytophthora list from protected horticulture UNVERIFIED. Its likely dominance in a hot Thai root zone below is inferred from its thermal biology alone.
Symptoms, in the order they appear
Seedlings (damping-off): seed rot before emergence, then a water-soaked lesion at the stem base, stem pinching, collapse. In a controlled test at 30 °C day / 25 °C night and 80% RH, P. aphanidermatum produced wilting, water-soaked stem lesions and basal collapse with near-complete infection within five days VERIFIED.
Established plants: fine feeder roots go first; roots become soft, brown and stubby; the root cortex sloughs off leaving the central thread when you pull; crown discolouration. Above ground: a midday wilt that recovers overnight, then one day does not; lower-leaf chlorosis; stunting VERIFIED.
Root rot or nitrogen deficiency?
| Signal | Oomycete root rot | Nitrogen deficiency |
|---|---|---|
| Distribution | Patchy — follows a drip line, a low spot, one bench or one tank, because zoospores move with water INFERRED | Uniform across everything on the same solution |
| Time course | Sudden; turgid to collapsed in 24–72 hours INFERRED | Gradual over 1–3 weeks |
| Daily pattern | Wilts at peak transpiration, recovers at night, then stops recovering VERIFIED | No wilt at all until terminal |
| Roots | Brown, soft, cortex slips off; feeder roots gone VERIFIED | White or cream and intact — often longer than normal, because a nitrogen-starved plant invests in roots |
| Response to feeding | None, or worse — you have just added more water INFERRED | Greens up in 5–10 days |
The decision rule: pull a plant and look at the roots before you change the feed. Wilting with white roots is a water, EC or VPD problem. Wilting with brown sloughing roots is an oomycete. Yellow with white intact roots is nutrition.
The numbers that decide it
| Species | Minimum | Optimum | Ceiling |
|---|---|---|---|
| P. aphanidermatum | 10 °C | 35–40 °C | — |
| P. ultimum (now Globisporangium ultimum) | 5 °C | 25–30 °C | cannot tolerate ≥35 °C |
| P. irregulare | 1 °C | 30 °C | 35 °C |
VERIFIED. The operational targets: hold the root zone and irrigation water at 18–22 °C; above 28 °C favours Pythium disease; keep substrate at about 70–75% wetness between irrigations rather than saturated VERIFIED.
Dissolved oxygen matters as much as temperature. Commercial minimum is 5 mg/L; growth is suppressed at 1–3 mg/L; an NFT channel can fall from 6.2 mg/L at the inlet to 2.9 mg/L at the outlet; and cooling a solution from 25 °C toward 18–22 °C raises oxygen solubility by about 15–20% VERIFIED.
The mechanism most growers miss. Warm water holds less oxygen and the root respires faster in it, so demand rises exactly as supply falls. Root tips go hypoxic, leak sugars and amino acids into the root zone, and those exudates are the chemical signal the zoospores swim toward. Root-zone temperature does not merely "favour the pathogen" — it manufactures the infection site INFERRED. That is why 18–22 °C is a disease target, not a growth target.
Thai season: hot season and the first rains
Isaan April mean maxima reach 36.1 °C at Surin and 36.6 °C at Nakhon Ratchasima VERIFIED, and the north-east recorded 43.6 °C in April 2020 VERIFIED. An unshaded substrate bag, a black tank or a shallow channel under that air temperature sits far above the 28 °C threshold and inside P. aphanidermatum's 35–40 °C optimum INFERRED. A second peak comes in the rainy season wherever drainage fails and saturation strips dissolved oxygen INFERRED. The cool season does not remove the problem — it shifts the species, because P. ultimum's optimum of 25–30 °C is then met instead INFERRED.
Control
1 · Cultural and environmental
Root zone 18–22 °C. Substrate 70–75% wetness, never saturated. Dry-back between irrigations. Then the Thai-specific additions INFERRED: insulate and shade nutrient tanks and every exposed run of irrigation pipe — a black pipe crossing open ground in April is a heat exchanger. Raise substrate off hot concrete. Never re-use unsterilised substrate. Sanitise propagation trays between cycles, because damping-off is overwhelmingly a propagation-hygiene disease.
2 · Water treatment and biologicals
Documented physical treatments: heat pasteurisation at 95–97 °C for 30 seconds, UV, ozonation, biofiltration VERIFIED. Slow sand filtration effectively reduced Phytophthora in recirculated nursery water, including after a deliberate pathogen switch and a simulated pump failure VERIFIED — it is robust, cheap and appropriate for a Thai farm.
Peroxide chemistry needs care: in one hydroponic trial the higher rate of a hydrogen peroxide / peroxyacetic acid product restricted lettuce growth while basil tolerated it, and overall algae suppression was not significant VERIFIED. Treat dosing as crop- and rate-specific and validate on a few plants first INFERRED.
Trichoderma harzianum and Bacillus subtilis are both listed in Thailand's Hazardous Substances List as Type 2 substances, meaning products containing them are registrable with DOA VERIFIED; both are FRAC group BM 02. Both are documented against P. aphanidermatum VERIFIED. Thailand's Department of Agricultural Extension publishes a Thai-language manual on mass-producing Trichoderma on-farm VERIFIED. We found no Thai field trial quantifying Trichoderma efficacy against Pythium in a Thai protected root zone specifically UNVERIFIED.
3 · Chemical, last
Propamocarb (including the hydrochloride) is listed in Thailand's Hazardous Substances List as a Type 3 substance under DOA responsibility VERIFIED; it is FRAC group 28 and is an oomycete-specific material. Metalaxyl / metalaxyl-M (FRAC 4) and fosetyl-aluminium (FRAC P 07) are the other standard groups, but we could not verify their Thai registration UNVERIFIED. Remember that ordinary broad-spectrum fungicides do nothing against oomycetes — the target biology is different. See the legal section.
Fusarium wilt and root rot
Thai โรคเหี่ยวฟิวซาเรียม · Vietnamese héo rũ Fusarium · Fusarium oxysporum, F. solani — true fungi (Ascomycota)
The trap in the name
Fusarium oxysporum is a species complex split into host-specific formae speciales VERIFIED. A generalist F. oxysporum population is near-universal in agricultural soils VERIFIED. So "there is Fusarium in my soil" does not mean "there is Fusarium that attacks my crop." A soil test that finds the genus tells you very little. We could not locate a Thai survey listing formae speciales in Thai protected horticulture UNVERIFIED.
What it looks like — and the one word that identifies it
One-sided. Yellowing and drooping begin on the lower leaves and are characteristically asymmetric — one side of a leaflet, one side of a leaf, or one side of the plant VERIFIED. Successive leaves yellow and wilt upward, the plant stunts, and a longitudinal cut near the stem base shows dark brown vascular discolouration running upward VERIFIED. That asymmetry is the field character separating it from bacterial wilt, southern blight and Verticillium wilt VERIFIED.
Fusarium, water stress, or magnesium deficiency?
| Signal | Fusarium wilt | Water stress | Magnesium deficiency |
|---|---|---|---|
| Symmetry | Asymmetric — one side of the plant or the midrib VERIFIED | Symmetric, whole canopy | Bilaterally symmetric interveinal INFERRED |
| Wilt | Recovers overnight at first, then permanently stops — and wilts even with wet substrate INFERRED | Recovers fully and repeatedly once watered | No wilt at all |
| Stem cut | Brown vascular ring or streaking VERIFIED | Clean white or green | Clean |
| Roots | May look reasonably intact — the blockage is in the xylem, not the cortex INFERRED | Intact, possibly dry | Intact |
The root observation is the trap: growers rule out disease because "the roots look fine". With Fusarium they can.
The numbers
28 °C soil and air temperature is the optimum for disease. Development is retarded above 34 °C and below 17–20 °C VERIFIED. Acidic soil favours it — pH 5.0–5.5 is favourable and raising to 6.5–7.0 reduces incidence — and ammonium nitrogen increases pathogen virulence while nitrate nitrogen decreases it, with nitrate-based feeding both raising yield and reducing wilted plants VERIFIED. That is an unusually actionable pair of levers.
Isaan's April mean is 29.9 °C VERIFIED — essentially the pathogen's optimum. Note the ceiling though: because development is retarded above 34 °C, the very hottest afternoons partially self-limit the disease, so expect symptom expression concentrated in late March and April and again at the transitions rather than at peak heat INFERRED.
Soil persistence — why rotation will not save you
The pathogen survives in infested soil for up to ten years as thick-walled chlamydospores VERIFIED. Once a Thai field block is infested, rotation alone will not clear it inside any normal planning horizon INFERRED. This is the strongest argument for moving to soilless substrate or to grafted plants rather than attempting to remediate the ground.
The nematode complex — important for Isaan
Root-knot nematode co-infection increases Fusarium wilt severity, documented across cotton, bean and watermelon VERIFIED. More importantly, Meloidogyne incognita allows Fusarium wilt of tomato to develop at or below the minimum temperature at which wilt normally occurs VERIFIED — the nematode widens the pathogen's environmental envelope, so the cool season stops protecting you. We could not retrieve exact magnitudes, only direction UNVERIFIED.
Relevance here: the light sandy soils typical of the Korat plateau are classic root-knot habitat, so open-field Fusarium wilt in Buriram and the surrounding provinces should be assumed to be a complex rather than a single-pathogen problem unless a nematode assay says otherwise INFERRED.
Control
1 · Cultural
Resistant varieties and rootstocks; 3–5 year rotation where the ground is not already heavily infested; lime to pH 6.5–7.0; only healthy transplants; avoid excess nitrogen and prefer nitrate over ammonium forms VERIFIED. Grafting onto resistant rootstock is the World Vegetable Center's standard tropical answer to soilborne disease, flooding and root-knot nematode VERIFIED.
2 · Soil treatment and biologicals
Solarisation reduces Fusarium inoculum, is documented by FAO for use inside plastic houses, and has demonstrated efficacy against F. oxysporum f. sp. lycopersici VERIFIED. The Thai hot season is the natural solarisation window — the same insolation that drives 36 °C April maxima is free energy for a four to six week tarped fallow INFERRED. Anaerobic soil disinfestation works too but its efficacy depends on soil temperature, amendment type, rate and C:N ratio VERIFIED.
Trichoderma is documented in biological control of both fungal and nematode disease VERIFIED, and T. harzianum is a Thai-listed Type 2 substance VERIFIED.
3 · Chemical, last
There is no curative chemical treatment for a plant already showing vascular wilt. Effort belongs in prevention. See the legal section for how to choose anything you do apply.
Powdery mildew
Thai โรคราแป้ง · Vietnamese bệnh phấn trắng · Erysiphales — obligate biotrophic fungi
Thailand has the highest diversity in the region
Southeast Asia has 14 genera and 96 powdery mildew species recorded, and Thailand has the highest diversity in the region VERIFIED. Erysiphe has the most species; Oidium — the name used where the sexual stage is unknown — has the broadest host range, covering cucurbits, legumes, brassicas, grapes, papaya, mungbean, soybean, roses, zinnias, rubber, teak and eucalyptus VERIFIED. Expect Golovinomyces, Podosphaera and Oidium in protected crops INFERRED.
The mildew that hides underneath
Leveillula taurica is the one to know about, because it behaves backwards. It develops inside the leaf. On the upper surface you see patchy yellowish or brownish blotches with no powder at all. The white sporulation is on the lower surface, emerging through the stomata. Leaf edges curl upward; badly affected leaves drop and expose fruit to sunscald. Secondary cycles run every 7–10 days VERIFIED. It is a global challenge in pepper production VERIFIED; we found no Thai record specifically UNVERIFIED, so treat it as a regional risk rather than a documented Thai one.
This is the single most misdiagnosed disease in the group. From above, Leveillula looks exactly like a magnesium or potassium deficiency, because the grower never turns the leaf over INFERRED. Rule: interveinal or blotchy chlorosis on an older leaf → turn the leaf over and look at the underside with a hand lens before you touch the feed.
Powdery mildew or a deficiency?
The sign of the pathogen is decisive: white mycelium is present, on the upper surface for the common mildews and the lower surface for Leveillula, and it rubs off on a fingertip INFERRED. Nutrient chlorosis rubs off nothing, is diffuse, follows the vein architecture bilaterally, and appears simultaneously across every plant on the same feed. Mildew appears as discrete circular colonies that enlarge radially and spread plant-to-plant along the prevailing airflow.
The exception that changes everything
Powdery mildew conidia do not require free water to germinate VERIFIED. Every other fungal pathogen in this guide does. Measured figures (on grape) VERIFIED:
- Relative humidity anywhere from 40% to 100% is sufficient for germination and infection.
- Infection occurs 15–32 °C, optimum 20–25 °C.
- Germination is suppressed above about 35 °C; above about 40 °C spores are killed.
- Latent period from infection to sporulation is about 7 days at the optimum.
- Free moisture, especially rainfall, is detrimental to conidial survival.
- Low, diffuse light favours development.
For Leveillula taurica on pepper: infects across 18–33 °C, slows above about 35 °C, and infection occurs under both high and low humidity VERIFIED. The regional review states plainly that Southeast Asian powdery mildews "thrive well even under warm and dry conditions" VERIFIED.
Why this demands the opposite management from everything else INFERRED, built directly on those verified numbers:
- You cannot dry your way out. 40% RH is already enough, so there is no achievable dehumidification target. Every other disease here is fought by drying the crop; this one is not.
- Your rain shelter creates the problem. Rain and free water are harmful to the conidia, so a roof removes the natural suppression an outdoor crop gets free during the monsoon.
- Your shade cloth makes it worse. Low diffuse light favours it, so the heavy shade installed for hot-season heat management simultaneously raises mildew risk.
- Temperature is the only real lever. Germination is suppressed above 35 °C and spores die above 40 °C — which is why an unshaded, uncooled Thai hot-season canopy effectively sterilises itself, and a cooled indoor room held at 20–26 °C sits precisely on the 20–25 °C optimum.
Thai season: the cool season, and any cooled room year-round
Isaan December and January means of 24.2–24.5 °C sit inside the 20–25 °C optimum, RH of 62–67% is far above the 40% floor, and there are 0.4–2.6 rain days per month to wash conidia off VERIFIED. November to February is the powdery mildew season in Thailand, and a temperature-controlled indoor room has it all year INFERRED.
Control
1 · Cultural and environmental
Because humidity control fails here, the levers are different INFERRED: maximise light penetration and avoid dense shading; open the canopy by pruning and wider spacing so no leaf sits in deep diffuse shade; maintain strong horizontal airflow so no still boundary layer forms; scout the undersides of older leaves weekly; remove and bag infected leaves while the colonies are still discrete circles, before they coalesce; avoid soft over-fertilised growth.
2 · Biological and nutritional
Silicon in the nutrient solution suppresses powdery mildew on greenhouse cucumber — but the suppression is inhibited at high temperature VERIFIED. So do not expect silicon to perform in a hot uncooled Thai house; test it in cool-season or cooled-room conditions, which is where the published effect was obtained INFERRED. Potassium silicate suppressed strawberry powdery mildew and has been evaluated on zucchini and gerbera VERIFIED. We could not retrieve the concentrations or the temperature at which silicon suppression failed UNVERIFIED, which is exactly the number that matters here.
Ampelomyces spp., a mycoparasite of powdery mildews, shows promise in Southeast Asia VERIFIED; Bacillus amyloliquefaciens induced systemic resistance against cucurbit powdery mildew VERIFIED, and Bacillus subtilis is a Thai-listed Type 2 substance VERIFIED (FRAC BM 02).
3 · Chemical, last
Sulfur (กำมะถัน) is listed in Thailand's Hazardous Substances List as a Type 3 substance VERIFIED and is FRAC group M 02 — a multi-site material with low resistance risk, and the classic powdery mildew answer. Note that sulfur is phytotoxic at high temperature, which is a real constraint in Thailand. Difenoconazole and tebuconazole are also Thai-listed Type 3 substances VERIFIED, both FRAC group 3 (DMI) — single-site materials that must be rotated with a different FRAC group. Potassium bicarbonate (FRAC NC) is widely used elsewhere but we could not verify Thai listing UNVERIFIED. See the legal section.
Grey mould (Botrytis)
Thai โรคราสีเทา · Vietnamese mốc xám · Botrytis cinerea — a necrotrophic fungus
What it attacks
It preferentially attacks tender new growth, freshly injured tissue, and ageing or senescent tissue, and insect feeding wounds are entry points VERIFIED. It typically starts on a spent flower, a fallen petal, damaged tissue or overripe fruit rather than on a healthy surface VERIFIED. It spreads on air currents and splashing water and survives long-term as resting structures VERIFIED. Inside a dense flowering canopy the rot can be well advanced internally before anything is visible from outside INFERRED.
Three-way distinction: grey fuzzy sporulation with an advancing margin into living tissue = Botrytis. Wet, slimy, foul-smelling with no fuzz = bacterial soft rot. Dry and static on a spent flower = ordinary senescence INFERRED.
The numbers
Measured on greenhouse floriculture crops VERIFIED: infection temperature range about 13–24 °C, optimum colonisation about 16–24 °C, RH ≥ 85%, and a film of moisture for 8 to 12 hours. Unlike powdery mildew, Botrytis genuinely requires free water or near-saturation.
Is Botrytis actually a problem in hot lowland Thailand? Mostly no
We could not find a published record of Botrytis cinerea causing economic loss in lowland Thai field or plastic-house horticulture UNVERIFIED; first-report searches returned Mauritius, Kazakhstan, China and Canada. Since the species is globally ubiquitous, the absence of a Thai paper is weak evidence. But the mechanism is clear, read against verified Thai climate normals INFERRED:
| Setting | Risk | Why |
|---|---|---|
| Lowland, hot season (Mar–May) | Low | Isaan night minima of 23.3–25.2 °C sit at or above the ~24 °C infection ceiling; daytime is far above |
| Lowland, rainy season (May–Oct) | Low to moderate | Humidity is satisfied comprehensively (Surin September 83.7% RH, 14.8 rain days) but night minima of 24.3–25.2 °C are still at the ceiling. The rainy-season rot you actually get in lowland Thailand is far more likely bacterial soft rot or anthracnose than Botrytis — this is the most commonly imported mistake from European guides |
| Lowland, cool season (Nov–Feb) | This is the window | Night minima of 18.5–21.0 °C are inside the optimum, and an 11.6 °C December day/night swing at Surin produces dew and condensation |
| Highland (northern uplands, Phetchabun, Loei) | Expect it as a primary pathogen | Sits inside the 16–24 °C optimum for much of the year INFERRED; no Thai highland survey located UNVERIFIED |
| Sealed, cooled indoor room, anywhere | Highest of all | A room held at 20–26 °C is inside the published optimum. If RH goes above 85% at lights-off, or surfaces fall to dew point when cooling cycles, all three conditions are met every night regardless of the monsoon outside |
Bottom line: in Thailand this is a cool-season, highland and indoor-room disease. Budget attention for it November to February and in any climate-controlled room, and do not import a European year-round Botrytis protocol into a hot-season open house INFERRED.
Control
Verified greenhouse practice: reduce humidity by combined heating and ventilation so a complete air exchange takes 5 to 10 minutes; install horizontal airflow fans to prevent condensation; water early in the morning so moisture evaporates as temperature rises; remove all plant debris and never store it in the house; space plants for circulation; avoid over-fertilisation, which produces succulent susceptible growth; and control insects, because their wounds are the entry points VERIFIED.
Humidity engineering targets VERIFIED: start dehumidifying at about 85% RH; keep RH below 90%, because at 90% only a slight temperature drop reaches dew point; target VPD 8–10 mbar; dehumidify when VPD falls to 4.5 mbar or below and humidify above 12.5 mbar; note that at about 12 mbar stomata are already around 50% closed and photosynthesis is being limited. Use thermal screens to stop radiative cooling of plant surfaces at night, and gradual pre-dawn temperature ramps to avoid sudden morning condensation. Local condensation still occurs from uneven heat distribution and the thermal mass of the plants themselves even when the bulk RH reading looks acceptable.
The highest-value Thai-specific instruction in this section INFERRED: the dangerous moment is not the wettest hour, it is the fastest temperature drop. A sealed room or closed house at 80% RH will condense on leaf surfaces the moment leaf temperature falls below dew point — which happens at lights-off, before dawn, and when a monsoon squall drops the outside temperature six to eight degrees in twenty minutes. Manage the rate of change, not just the setpoint.
Biological options with published evidence: Clonostachys rosea and Metarhizium anisopliae against tomato grey mould, and Bacillus secreted metabolites against foliar disease, backed by a 22-year meta-analysis of Bacillus biocontrol VERIFIED. For chemical options see the legal section; Botrytis is notoriously quick to develop resistance, so FRAC group rotation is not optional.
Fungal leaf spots
Thai โรคใบจุดจากเชื้อรา · Vietnamese đốm lá do nấm · Ascomycota
Which genera actually matter in Thailand — and which do not
1 · Colletotrichum — anthracnose, the dominant one
Ten Colletotrichum species cause anthracnose of Capsicum across Asia. Recorded from Thailand: C. siamense (common), C. fructicola, C. plurivorum and C. endophyticum VERIFIED. The most aggressive species regionally are C. javanense and C. scovillei, especially on non-wounded fruit — they do not need a wound VERIFIED. C. siamense was described from Thailand and also causes leaf spot on rubber in the south and anthracnose on ornamental aroids here VERIFIED; its Thai host range is effectively "most broad-leaved horticulture" INFERRED.
2 · Corynespora cassiicola — target spot, confirmed in a Thai glasshouse
First reported on hydroponically grown lettuce in glasshouses in three districts of Songkhla province, June–August 2016. Lesions were brown to dark brown with light and dark concentric rings, circular to irregular, 5–15 mm across; severe infection caused leaf blight and killed young plants. The paper notes explicitly that regular misting and spraying inside the enclosed glasshouse dispersed the spores VERIFIED.
This is the most directly transferable record in the whole guide — same country, same enclosed hydroponic system, same rainy-season timing. Treat it as a first-tier risk INFERRED.
3 · Cercospora and Alternaria
Cercospora is present and economically significant regionally VERIFIED, though we found no Thai protected-horticulture record UNVERIFIED. Alternaria brassicicola black spot is an active research target at Kasetsart University VERIFIED; Alternaria matters here chiefly on senescing or stressed lower foliage and post-harvest INFERRED.
4 · Septoria — probably not your problem
We found no evidence that Septoria is significant in Thai tropical horticulture UNVERIFIED. These leaf spots are predominantly temperate-adapted. Deprioritise the genus relative to Colletotrichum and Corynespora — and treat its prominence in a guide as a signal that the guide is a temperate document you should not be following INFERRED.
How a fungal leaf spot progresses, and how to tell it from nutrition
A pinpoint water-soaked or chlorotic fleck, usually on lower and older leaves because they are closest to splashed soil and stay wet longest; expansion into a defined lesion with a distinct margin, often with concentric rings or a chlorotic halo; coalescence into blight; defoliation from the bottom up; and for anthracnose, progression onto flowers and fruit with sunken lesions and pink-orange spore masses in humid conditions INFERRED. Alternaria solani gives characteristic target-like concentric rings and lesions visible in as little as five days VERIFIED.
| Signal | Fungal leaf spot | Nutrient disorder |
|---|---|---|
| Lesion margin | Discrete, hard margin, often with a halo or concentric rings | Diffuse gradient, no defined edge |
| Sign of the pathogen | Fruiting structures visible at 10–20× in the lesion centre — a pink spore mass for anthracnose, dark conidiophores for target spot | Nothing, at any magnification |
| Distribution on the plant | Random scatter, denser where the canopy stays wet and near splash height | Follows leaf age strictly — oldest or youngest |
| Progression | New lesions after every wet event; the count increases | Static, or slowly generalising |
| Distribution in the block | A focus with a gradient outward from one plant | Uniform across every plant on the same feed |
The specific trap: potassium-deficiency marginal scorch versus a coalescing marginal blight. Potassium deficiency gives a continuous necrotic margin following the whole leaf edge, symmetric, with a yellow transition band. A blight is discontinuous — it is merged discrete lesions, and you can usually still make out the individual round outlines inside it INFERRED.
The numbers, and why the rainy season is anthracnose season
Tropical Colletotrichum measured in Guadeloupe: optimum 25–28 °C, maximum 36 °C VERIFIED. Cercospora beticola: optimum 25–35 °C, optimum RH 90–95%, and fewer than 11 hours of leaf wetness prevents or greatly reduces infection VERIFIED. Alternaria solani: optimum 28–30 °C, requires RH ≥90% or free water VERIFIED.
Isaan rainy-season means are 27.4–29.5 °C VERIFIED — dead centre of all three. Surin has 11–15.5 rain days a month with 74.8–83.7% mean RH from May to September VERIFIED, so the 11-hour wetness requirement is trivially met. And every one of these genera is splash-dispersed VERIFIED, with monsoon rain acting as the spore transport system. Temperature optimum, wetness duration and dispersal energy arrive together. That triple coincidence is why the wet tropics are anthracnose country INFERRED.
The exception is Corynespora, which does not need outdoor rain — the Thai record came from an enclosed glasshouse and was dispersed by misting VERIFIED. A rain shelter removes the splash driver for anthracnose and Cercospora but does nothing about target spot if you mist overhead INFERRED.
Control
1 · Cultural — this is where the wins are
Verified practice: rotation (at least 2 years for Alternaria), mulching as a physical barrier against soil splash, drip irrigation to keep foliage dry, wider spacing and trellising for airflow, removal of infected lower leaves, elimination of susceptible volunteer plants VERIFIED. Tropical IPM standard practice adds healthy seedling production under 30–50 mesh netting, rotation with non-hosts, and burying infested fruit 15–45 cm deep VERIFIED.
Thai-specific and highest leverage INFERRED: a rain shelter with open sides is the single most effective intervention for this whole group during the monsoon, because it breaks splash dispersal and caps leaf-wetness hours while preserving the ventilation you need for heat. Switch overhead misting to drip or sub-canopy before the rains start, not after. Strip the lowest 30 cm of leaves so the splash zone is bare stem.
2 · Biological — and Thailand has real numbers here
Bacillus subtilis against chilli anthracnose, tested at Suranaree University of Technology in Nakhon Ratchasima — that is Isaan, the same region as the farm: about 55% antagonism in vitro, and under net-house conditions 86–100% reduction in anthracnose severity on leaves and 49–74% on fruit VERIFIED. This is the best-evidenced biological result in this guide and it was obtained in the right region on the right disease.
At Kasetsart University, Trichoderma asperellum KUFA 0111 and T. harzianum KUFA 0141 gave significant biocontrol of anthracnose; against Alternaria brassicicola they gave 26.7–31.1% disease reduction applied preventatively but only 22.6–24.1% curatively VERIFIED — clearly better as a preventative than as a rescue, which is the pattern for almost every biological. Thai indigenous T. harzianum also controlled Phytophthora palmivora leaf fall in Thai rubber, evidence that Thai-isolated strains perform under Thai field conditions VERIFIED.
Thailand's DOA Plant Protection Research and Development Division maintains a public catalogue of biological products (ชีวภัณฑ์) — the authoritative Thai list VERIFIED, though we could not retrieve its contents UNVERIFIED. It is worth a visit in person.
3 · Chemical, last
Chlorothalonil is a Thai-listed Type 3 substance VERIFIED, FRAC group M 05 — a multi-site protectant with low resistance risk, used preventatively before a wet period rather than curatively after one. Difenoconazole and tebuconazole are Thai-listed Type 3 substances VERIFIED, FRAC group 3. Azoxystrobin (FRAC 11) and mancozeb (FRAC M 03) are standard elsewhere but we could not verify Thai registration UNVERIFIED. Anthracnose in the tropics is a heavy spray programme if you rely on chemistry alone, which is precisely why the roof and the mulch matter more. See the legal section.
Bacterial leaf spot and soft rot
Thai โรคใบจุดจากแบคทีเรีย and โรคเน่าเละ · Vietnamese đốm lá vi khuẩn and thối nhũn
Bacteria are not fungi and they behave differently. They have no mycelium, no spores and nothing that blows about in dry air. They move in water — rain splash, irrigation water, contaminated tools, and the hands and clothes of anyone walking through a wet crop VERIFIED. Every control decision follows from that.
Bacterial leaf spot — Xanthomonas
At least four taxa cause bacterial leaf spot of chilli and pepper — Xanthomonas euvesicatoria pv. euvesicatoria, X. euvesicatoria pv. perforans, X. hortorum pv. gardneri and X. vesicatoria — widely distributed in the tropics and subtropics, surviving in seed and crop residue, and causing up to 66% losses VERIFIED. Which pathovar predominates in Thailand specifically is UNVERIFIED; no Thai survey was located.
Symptoms in sequence: small water-soaked spots → black or dark brown angular lesions, often stopping at the veins → a yellow halo → coalescence, shot-holing, defoliation → raised scabby lesions on fruit VERIFIED. The water-soaked early phase is the bacterial signature: hold the leaf up to the light and a young lesion is translucent and greasy-looking, not dry INFERRED.
The number that matters. Using the best-quantified Xanthomonas wetness model available (measured on Prunus, so cite it honestly) VERIFIED: minimum 3–6 hours of leaf wetness for infection at optimal temperatures; optimum 28.9 °C; severe infection range 20–35 °C; 5 hours of wetness at 25–35 °C produces high disease severity, while at 20 °C it takes more than 10 hours.
Isaan's rainy-season mean is 27.4–29.5 °C VERIFIED — essentially the optimum — and in that band the requirement collapses to five hours of wetness. Five hours of leaf wetness in a Thai August is not a risk event, it is every night INFERRED. That is why bacterial leaf spot here is a rainy-season certainty rather than a rainy-season possibility.
Bacterial soft rot — Pectobacterium and Dickeya
Cardinal temperatures determined in vitro VERIFIED:
| Organism | Minimum | Optimum | Maximum |
|---|---|---|---|
| Pectobacterium carotovorum subsp. carotovorum | 20 °C | ~31 °C | 38 °C |
| P. carotovorum subsp. brasiliense | 20 °C | 31 °C | 38 °C |
| P. wasabiae | 20 °C | 29 °C | 34 °C |
| Dickeya dadantii | 21 °C | 32 °C | 38 °C |
| Dickeya solani | 23 °C | 34 °C | 41 °C |
This is the most important table on this page for a Thai grower. Optima of 31–34 °C with ceilings of 38–41 °C mean the soft rot bacteria are the only pathogen group in this whole guide whose optimum sits fully inside the Thai hot- and rainy-season temperature envelope. Everything else here is fighting the Thai climate at the top of its range. Soft rot is at home in it. "Tropical heat plus rain equals collapse into slime" is not folklore — it is this table INFERRED.
One conflict to flag honestly: a widely used extension wiki gives P. carotovorum as thriving at 20–25 °C, contradicting the peer-reviewed in-vitro determination of about 31 °C VERIFIED both statements. We weight the peer-reviewed cardinal-temperature study and treat 28–34 °C as the operational Thai danger band INFERRED.
Soft rot requires high moisture; extended soil saturation facilitates infection; it penetrates via wounds — petiole wounds travelling to the stem, post-harvest bruising, and injuries created by earlier fungal disease; it disperses via rain splash, contaminated irrigation water, insects, and contaminated tools or media; it is facultatively anaerobic; and it survives in infected debris, contaminated potting medium and unsanitary storage VERIFIED. Symptoms run: a small dark water-soaked spot at a wound or a leaf axil where water sits → rapid maceration into soft structureless mush within 24–48 hours at 28–34 °C → a foul putrid odour → collapse of the stem or petiole INFERRED. No Thailand-specific survey on horticultural crops was located; the nearest confirmed record in the region is pepper fruit soft rot in Malaysia UNVERIFIED for Thailand.
How monsoon rain drives both
Built from the verified components INFERRED:
- Bacteria have no wind-dispersed spore. They travel as cells inside water droplets.
- A raindrop striking an infected lesion atomises into hundreds of daughter droplets carrying bacterial cells a metre or more, so one infected plant becomes a point source with a downwind gradient during a single storm.
- Monsoon rain also wounds the crop — leaf abrasion, petiole tearing, stem flexing. Wind-driven rain therefore delivers the inoculum and the entry wounds in the same event. There is no equivalent coupling for fungi.
- Rain saturates canopy and soil, giving Xanthomonas the five hours of wetness it needs at Thai temperatures, and giving Pectobacterium the water congestion it excels in.
- Workers moving through a wet crop are a splash vector equivalent to rain.
The operational rule that matters more than anything you can spray: do not touch, prune, train, scout or harvest a wet crop.
Control
Break the splash path: rain shelter or plastic roof, mulch the soil surface, strip the lowest leaves, drip or sub-canopy irrigation only. Cap leaf-wetness hours: irrigate early morning, ensure the canopy is dry before nightfall, 5–10 minute air exchange, horizontal airflow fans VERIFIED. Eliminate wounds: prune only in dry conditions and only when tissue will dry within a few hours, disinfect tools between plants, and control chewing and rasping insects whose feeding wounds are entry courts VERIFIED. Seed and propagation hygiene: Xanthomonas survives in seed and residue VERIFIED, so start from clean material and never propagate from a symptomatic mother plant. Drainage: avoid extended soil saturation; raised beds are standard tropical practice. Rogue and remove infected plants in dry weather, bagging them on the spot rather than carrying them open through the crop.
Biological and non-chemical: bacteriophage deployment is a reported biological control for bacterial leaf spot, and stable host resistance is identified as the critical component of integrated management VERIFIED. Bacillus secreted metabolites control foliar disease VERIFIED. We found no quantified efficacy data for Bacillus or phage against soft rot under tropical field conditions UNVERIFIED.
Chemical: copper hydroxide and copper oxychloride are Thai-listed Type 3 substances VERIFIED, FRAC group M 01. Copper is the conventional bacterial material worldwide, it is protectant only — it does nothing to a plant already systemically infected — and copper resistance in Xanthomonas is well known globally. There is no chemical cure for bacterial soft rot. See the legal section.
Virus diseases spread by thrips and whitefly
Thai โรคไวรัส · Vietnamese bệnh virus · Orthotospoviruses and begomoviruses
Which viruses are actually in Thailand — and the big correction
Three orthotospovirus species have been characterised from Thailand, from tomato, pepper, cucurbits, peanut and Physalis: Watermelon silver mottle virus (WSMoV), Capsicum chlorosis virus (CaCV) including a novel tomato-necrosis strain, and Melon yellow spot virus (MYSV). The vectors recovered from the same fields were Thrips palmi and Scirtothrips dorsalis VERIFIED. A fourth, Tomato necrotic ringspot virus (TNRV), was first isolated in Thailand VERIFIED. Ceratothripoides claratris is a confirmed Thai vector of CaCV and TNRV VERIFIED.
Tomato spotted wilt virus was NOT among the tospoviruses identified in the principal Thai survey VERIFIED. Nearly every English-language greenhouse guide leads with TSWV and western flower thrips. In Thailand the operative viruses appear to be CaCV, WSMoV, MYSV and TNRV, and the operative vectors are Thrips palmi, Scirtothrips dorsalis and Ceratothripoides claratris INFERRED. We did not find a definitive statement that TSWV is absent from Thailand, so treat this as "not detected in the main Thai survey", not as proven absence UNVERIFIED.
Begomoviruses, quantified. A survey of 585 samples from 17 provinces between January 2015 and June 2017 found 57% begomovirus-positive, identifying 15 known species plus one novel candidate VERIFIED: Tomato yellow leaf curl Thailand virus predominant in tomato, Pepper yellow leaf curl Thailand virus predominant in pepper, Tomato leaf curl New Delhi virus in loofah, bitter gourd, cucumber and melon, Squash leaf curl China virus in pumpkin and wax gourd, and a novel Melon yellow mosaic virus candidate from melons in north-eastern Thailand. The vector is Bemisia tabaci. Whitefly-transmitted leaf curl in tropical Asia can reach 100% incidence with yield losses often exceeding 90% VERIFIED.
Virus, or magnesium and iron deficiency?
| Signal | Virus | Magnesium deficiency | Iron deficiency |
|---|---|---|---|
| Green/yellow boundary | Sharp, irregular, mosaic-like — islands of dark green against pale with abrupt edges | Gradual gradient | Gradual |
| Symmetry within the leaf | Asymmetric — one half of a leaf can differ from the other | Bilaterally symmetric interveinal | Bilaterally symmetric |
| Leaf age affected | Youngest, newly expanding leaves most distorted | Oldest leaves first, strictly | Youngest leaves first |
| Leaf shape | Distorted — curling, cupping, strapping, blistering, reduced lamina | Normal shape | Normal shape |
| Distribution in the block | Individual scattered plants, often at the edge first, spreading over weeks | All plants on the same feed at once | All plants, or all in a high-pH zone |
| Response to correction | No response to foliar magnesium or chelated iron | Greens up in 7–14 days | Greens up fast with chelate |
INFERRED throughout, from standard virology and the verified deficiency patterns. The two decisive tests:
- Distribution. Nutrition is a system property — it hits every plant on the same tank at once. Virus is a plant property — it hits scattered individuals and spreads. If exactly one plant in twelve is yellow, it is not the feed.
- Distortion. Nutrient deficiencies change leaf colour. Viruses change leaf shape and colour together. Cupping, strapping, blistering, reduced leaflet size and shortened internodes mean virus.
Third confirmation: correct the nutrient hypothesis properly and wait 10–14 days. A deficiency responds. A virus does not, and by then it has spread to the neighbours.
The transmission detail that makes thrips timing counterintuitive
Measured for Watermelon silver mottle virus with Thrips palmi VERIFIED:
- The virus is acquired mainly at the first-instar larval stage and then transmitted mainly by the adults that ingested it as larvae.
- Adults feeding on an infected plant for the first time are essentially incompetent — transmission efficiency after adult-stage exposure was only 6.7%.
- Latent period about 10 days between larval acquisition and infectious adult.
- No transmission to offspring — the virus does not pass through the egg.
- The virus replicates inside the vector, infecting the salivary glands, digestive tract and Malpighian tubules.
What that means on the farm INFERRED:
- An adult thrips flying in from outside cannot make your plants sick on its own unless it acquired the virus as a larva somewhere else. At 6.7%, adult acquisition is nearly a dead end.
- The dangerous thrips is the one that hatched on an infected plant inside your crop. The moment you have one infected plant, your resident larval population becomes the epidemic engine. Roguing is not tidiness — it is breaking the acquisition step.
- The ten-day latent period is your intervention window. Rogue the infected plant and suppress larvae before that cohort matures and you interrupt the cycle. Wait, and you get a synchronised wave of infectious adults.
- Larval control matters more than adult control for virus, even though adult control matters more for feeding damage. Sticky traps monitor adults and tell you about immigration; they do not measure your virus risk, which is sitting on the undersides of leaves.
The non-negotiable virus rules
- A virus-infected plant cannot be cured. There is no treatment, biological or chemical. Nutrition will not fix it, biologicals will not fix it, time will not fix it.
- Roguing is the response. Remove infected plants promptly, bag them where they stand, and carry them out sealed. Never walk a symptomatic plant through the crop uncovered.
- Vector control is the prevention — exclusion, then monitoring, then suppression, in that order.
- Never take cuttings or select mother plants from a block with any virus incidence. Begomoviruses in particular are efficiently perpetuated through vegetative propagation.
- Volunteer and alternate host plants are reservoirs; clean cultivation around the structure is explicit tropical IPM guidance VERIFIED.
The strongest result in this entire guide — and it was measured in Thailand
UV-blocking plastic films and nets on greenhouse tomatoes, tested at the Asian Institute of Technology in Bangkok from March to September 2005 — the Thai hot and rainy seasons — against Bemisia tabaci, Ceratothripoides claratris and Aphis gossypii VERIFIED:
Virus infection reached 96–100% under UV-transmitting greenhouses, against 6–10% in greenhouses where UV was blocked.
Tospoviruses, primarily Capsicum chlorosis virus, were the major infections; symptom appearance was considerably delayed under UV-blocking roofs; fewer whiteflies, aphids and thrips entered; and no significant differences in temperature or humidity were found between the four tunnel types — so the UV-blocking material cost nothing in microclimate.
Insects navigate on ultraviolet light. Removing the UV signal disorients immigrating vectors at the boundary of the structure. A drop from 96–100% to 6–10% virus incidence, obtained in Bangkok, with no heat penalty, is not matched by anything else in this guide INFERRED. If you are choosing covering material for a Thai structure, this is the decision that matters most.
Insect exclusion netting of 30–50 mesh is the tropical standard for protected seedling production VERIFIED, with the honest caveat from the thrips section that ordinary net in that range reduces rather than eliminates thrips. And the netting trade-off in Thailand is real: finer mesh excludes thrips but cuts the ventilation rate, which in a hot-season house pushes internal temperature into crop-damaging territory VERIFIED. The engineered answer demonstrated for north-eastern Thailand was forced ventilation: a CFD-optimised greenhouse in Nong Bua Lamphu using a 15° air-inlet angle with three bottom-positioned exhaust fans held the interior at 35.72 °C — 7.88 °C below ambient VERIFIED. Design rule: if you are going to screen, you must force-ventilate INFERRED. Screening without fans converts a heat problem into a heat and disease problem.