Steps To Diagnose Fungal Diseases On New Hampshire Vegetable Plants

Diagnosing fungal diseases on vegetable plants in New Hampshire requires a systematic approach that combines careful observation, basic field tests, proper sample collection, and, when necessary, laboratory confirmation. Weather patterns in New Hampshire, with cool, wet springs and warm, humid summers in many locations, create favorable conditions for a range of true fungi and fungal-like pathogens. This article provides step-by-step guidance you can apply at the garden or farm scale, practical diagnostic tips, and management recommendations tied to specific diagnoses.

Understand the scope: fungi versus fungal-like pathogens

Not every disease that looks fungal is caused by a true fungus. Oomycetes (water molds) such as Phytophthora and Pythium, and other soil-borne organisms, cause similar symptoms but have different biology and control needs. For practical on-farm diagnosis, treat “fungal” and “fungal-like” diseases together until laboratory testing clarifies the pathogen, but remember that fungicide choices and cultural controls may differ by organism group.

Why the distinction matters

True fungi generally produce spores and mycelium and respond to a different set of fungicides than oomycetes. For example, metalaxyl-type products are active on many oomycetes but not on most true fungi. Cultural controls such as improving drainage are vital for oomycetes. Knowing whether a pathogen is a fungus or an oomycete informs effective treatment planning.

Step 1: Observe and document symptoms carefully

Good diagnosis starts with careful observation and record keeping. Before removing samples, record the disease pattern, timing, and environmental context.

Observe which crops and varieties are affected, and note whether symptoms appear in patches, along low areas, or randomly.
Record the age and stage of plants when symptoms first appeared.
Note recent weather: number of rainy days, heavy dews, irrigations, temperature ranges, and soil wetness.
Document soil type, drainage, previous crop, and any recent field operations such as cultivation or harvest.

Symptoms to document by plant part:

Leaves: spots (shape, color, margin), chlorosis, necrosis, concentric rings, powdery coatings, downy fuzzy growth on undersides.
Stems: lesions, cankers, girdling, white cottony mycelium, black sclerotia.
Fruit: sunken lesions, rot texture (dry vs soft), sporulation, concentric rings, slime.
Roots: brown to black decay, rotting, root galls, stunting, discolored vascular tissue.

Take clear photographs showing whole plants, a close-up of symptomatic tissue, and the general field distribution. Note the date and time for each photo.

Step 2: Field diagnosis and simple tests

Field diagnosis can be surprisingly accurate with a few simple checks and minimal equipment.

Use a hand lens (10x or 20x) to look for spores, sporulation, fungal fruiting bodies, or powdery coatings.
Check leaf undersides for downy mildew or sporulation that may only appear on the lower surface.
Smell: some rots (Sclerotinia, soft rots) have characteristic odors. While subjective, this can help prioritize samples.
Perform a cut test: slice stems or fruit to inspect internal discoloration or vascular browning (indicative of wilt pathogens).
Press or rub suspicious tissue on white paper. Some fungal spores or rust pustules will leave colored marks; powder will smear.

When to suspect soil- or waterborne pathogens:

Patchy areas in low-lying, poorly drained spots following prolonged wet weather.
Rapid seedling collapse in cool, wet soils (Pythium, Phytophthora, Rhizoctonia).

When to suspect airborne pathogens:

Lesions with concentric rings, scattered spotting on foliage, or powdery coatings appearing under humid conditions but in well-drained soils (Alternaria, Cercospora, powdery mildew).

Step 3: Collecting and preparing samples for diagnosis

Correct sampling increases the chance of a definitive diagnosis from a diagnostic lab or extension service.

Collect multiple samples representing the range of symptoms (early, mid, advanced) and from different affected areas.
Include entire plants when possible for root and stem inspection; if not practical, include symptomatic leaves, stems, fruits, and a bit of adjacent soil.
Place samples in clean paper bags or cardboard boxes. Avoid airtight plastic bags for warm, moist tissue because they promote decay and alter diagnostic signs.
Keep samples cool and deliver to a diagnostic lab within 24 to 48 hours. Refrigerate if transport will be delayed but do not freeze.
Label each sample with date, location in the field, crop, variety, and brief history of symptoms and management practices.

If submitting samples is not feasible, preserve representative photos and detailed notes to share with advisors.

Step 4: Laboratory and advanced diagnostics

When field tests are inconclusive or when control decisions hinge on a precise identity, use diagnostic labs for confirmation. Typical laboratory methods include:

Microscopic examination: distinguishes spore types, hyphal structures, and signs of oomycetes versus true fungi.
Culture and isolation: growing the pathogen on selective media yields colonies that can be identified by morphology and sporulation patterns.
Serological tests: antigen-antibody assays can detect some specific pathogens quickly.
Molecular diagnostics: PCR or DNA sequencing provides species-level identification and can detect pathogens that are difficult to culture.
Pathogenicity tests: inoculating healthy plants under controlled conditions confirms whether an isolated organism causes the observed disease.

When submitting samples, provide as much field information as possible and ask the lab to report on the organism group (fungus, oomycete, bacteria) and species-level identification where available. Ask whether the lab can test for fungicide resistance if that question is relevant to management decisions.

Step 5: Interpret results and determine management actions

Match diagnosis to management that is effective for that organism group and crop situation. Use an integrated approach combining cultural, biological, and, when necessary, chemical controls.
Cultural and physical controls to prioritize:

Sanitation: remove and destroy infected debris at the end of the season. Fungal sclerotia, spores, and infected seed are common carryovers.
Rotation: rotate away from crops susceptible to the same pathogen for at least two to three years when feasible.
Resistant varieties: select varieties with documented resistance to specific diseases where available.
Water management: improve drainage, avoid overhead irrigation that wets foliage during evenings, irrigate in the morning to promote rapid drying, and space plants to increase air movement.
Soil amendments and pH: certain pathogens favor low pH or compacted soils; correct soil conditions when practical.

Chemical controls and important caveats:

Use fungicides as part of a program, timed to disease risk (environmental conditions and crop stage).
Select products active against the confirmed pathogen group. For example, oomycetes require different chemistries than many true fungi.
Rotate modes of action to minimize resistance development; follow label instructions for application rates and intervals.
Follow all label restrictions for crops, preharvest intervals, and environmental precautions.

Biological and cultural products:

Consider biological fungicides, biostimulants, and soil-health practices that reduce disease pressure. Their efficacy varies; use as part of an integrated program rather than as sole control for severe outbreaks.

Practical profiles: common New Hampshire vegetable fungal diseases and how to recognize them

Below are concise diagnostic clues and management takeaways for several diseases commonly encountered in New Hampshire vegetable production.
Powdery mildew (true fungi, many hosts)

Symptoms: white to gray powdery fungal growth on upper leaf surfaces, sometimes stems and fruit; yellowing and leaf distortion.
Conditions: warm days, cool nights, high humidity but not necessarily free water on leaf surfaces.
Diagnosis tips: powder rubs off; hyphae and chains of conidia visible under 10x hand lens.
Management: choose resistant varieties, increase air movement, remove infected tissue, apply targeted fungicides when needed.

Downy mildew (oomycete; hosts include cucurbits, lettuce)

Symptoms: angular yellow to brown lesions on upper leaf surface; gray to purple downy sporulation on the leaf underside under humid conditions.
Conditions: cool, wet weather, persistent leaf wetness.
Diagnosis tips: turn leaf over in humid conditions to see downy growth; oomycete-specific fungicides needed.
Management: improve drainage and spacing, avoid overhead irrigation at night, use resistant varieties and protectant/curative oomycete-active products guided by monitoring.

Early blight (Alternaria solani; solanaceous crops like tomato)

Symptoms: dark, target-like lesions with concentric rings on leaves and fruit; lower leaves often affected first.
Conditions: warm, wet weather and frequent leaf wetting.
Diagnosis tips: characteristic bullseye patterns; make sure to inspect stems and fruit.
Management: sanitation, mulching to reduce soil splash, rotation, and protectant fungicides; remove heavily infected lower leaves.

Late blight (Phytophthora infestans; potato and tomato; an oomycete)

Symptoms: water-soaked lesions that rapidly turn brown or black, white sporulation on undersides in humid conditions, rapid collapse of foliage.
Conditions: cool, wet weather with high humidity and storms.
Diagnosis tips: rapid onset and severe spread distinguish late blight; submit samples quickly for confirmation because of regulatory concerns and the need for aggressive control.
Management: remove infected plants, avoid saving seed tubers from infected fields, and apply oomycete-specific fungicides preventatively during high-risk periods.

White mold / Sclerotinia stem rot (Sclerotinia sclerotiorum)

Symptoms: white, cottony mycelium on stems and crown; black hard sclerotia inside tissue or on surface.
Conditions: cool, wet spring and early summer, dense canopies that retain humidity.
Diagnosis tips: look for white mycelial mats and black sclerotia; these are diagnostic.
Management: deep burial or removal of sclerotia-bearing debris, wider row spacing, and targeted fungicide applications when necessary.

Fusarium and Verticillium wilts (soil-borne fungi)

Symptoms: one-sided wilting, yellowing of lower leaves, gradual plant decline; brown to black vascular discoloration on stem cut surface.
Conditions: favored by warm soils for Fusarium and more moderate for Verticillium; often long-lived in soil.
Diagnosis tips: vascular browning in stems and roots is key; culture or molecular tests confirm species.
Management: plant resistant cultivars, solarize soil where feasible, rotate to non-host crops, and avoid transplanting into infested soils.

Rusts (Puccinia spp. and others)

Symptoms: orange to brown pustules on leaf surfaces that release powdery spores; can cause premature defoliation.
Conditions: moderate temperatures and high humidity favor many rusts.
Diagnosis tips: pustules are visible and often leave rust-colored dust on touch.
Management: remove volunteer hosts, select resistant varieties, and apply fungicides at threshold levels.

Record-keeping and follow-up monitoring

Keep a disease log each season documenting symptoms, weather conditions, management actions taken, and outcomes. This helps refine diagnosis over multiple seasons and improves timing of control measures. Re-inspect treated areas regularly, especially after weather events that favor disease, and be prepared to adjust tactics based on response and subsequent identification results.

Final practical takeaways

Start with careful observation: timing, weather, spatial pattern, and detailed photos are often half of the diagnostic work.
Use a hand lens, cut tests, and simple field checks to narrow possibilities before sending samples to a lab.
Collect and submit well-labeled samples promptly when lab diagnosis is needed.
Differentiate true fungi from oomycetes where possible because management differs.
Prioritize sanitation, resistant varieties, and cultural controls; use fungicides strategically and in rotation to reduce resistance risk.
Keep records and monitor after treatments so you can learn and improve management year-to-year.

Diagnosing fungal diseases is a stepwise process that blends observation, practical field tests, and laboratory confirmation when necessary. In New Hampshire, the seasonal weather patterns make prompt and informed action essential to protecting vegetable crops. Applying these steps will improve diagnosis accuracy and result in more effective, sustainable disease management.