Downy Mildew Three Sprays Later: Resistance, Residues and Rain
A grape grower in Niphad taluka has 1 ha of Thompson Seedless, harvested in about 45 days. The last three sprays, at 8-day intervals, were metalaxyl + mancozeb at the labelled dose. New oil spots with yellowish margins are still appearing on the upper leaf surface, and in the morning a white downy growth is visible on the corresponding lower surface. The weather for the coming week is 23–26°C with 90% relative humidity and two rainy days. Canes are dense, with poor bunch-zone ventilation. The farmer wants to repeat the same combination at double dose, add one more systemic 'for safety', and spray on the rainy day. The crop is contracted for export, the buyer has warned about residue limits and asked for harvest-interval details, and the grower wants to know what to spray tomorrow morning. He has 1 kg captan and 500 g dimethomorph in stock.
1. Confirm the diagnosis and explain why three sprays of metalaxyl + mancozeb failed.
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Diagnosis
The symptoms are downy mildew (Plasmopara viticola): yellowish 'oil spots' on the upper surface with matching white downy sporulation underneath, at 23–26°C and 90% RH. Powdery mildew would show ash-like powder on both surfaces, no oil spots and no downy growth below.
Why three sprays failed
- Resistance selection. Metalaxyl is a single-site, high-risk fungicide (FRAC group 4). Three consecutive sprays of the same active ingredient within a month is exactly the selection pressure that breeds resistant strains, and continuing new spots are consistent with it.
- Protectant, not eradicant. Mancozeb protects healthy tissue; it cannot cure an infection already inside the leaf. At 90% RH the incubation period is only about 4–5 days, shorter than the 8-day interval used.
- Timing against rain. Sprays applied before rain are washed off, leaving new growth unprotected at the moment of infection.
- Coverage. Dense canopies with poor bunch-zone ventilation mean the undersides of leaves and the bunches are under-sprayed - and the underside is where downy mildew establishes.
So the answer is not more of the same, and certainly not double the dose: doubling adds cost and residue without overcoming resistance.
2. What should be sprayed tomorrow morning, and what should the grower buy next? Cover dose logic, water volume, interval and residue/PHI handling for an export crop.
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Tomorrow morning, before the rain
- Use the captan in stock as the protectant partner plus the dimethomorph for tissue already infected: captan 50% WP at about 2.0–2.5 g/L with dimethomorph 50% WP at about 1.0–1.25 g/L, in 500–700 L water per ha for this canopy, with a sticker. Confirm the exact registered doses on the labels.
- Dimethomorph is FRAC 40: always mix it with a contact partner, never use it alone, and never use it twice in a row.
- Spray in the morning so the deposit dries before rain. Do not spray on the rainy day itself; if rain falls within a few hours of spraying, respray after it clears.
- Next round, switch to a different mode of action - for example cymoxanil + mancozeb (FRAC 27 + M3), or fosetyl-Al (FRAC 33) - alternating with a contact protectant.
Residue and harvest interval (critical for this crop)
- The crop is export-bound and 45 days from harvest. Check the buyer's MRL list and the label pre-harvest interval for each product before it enters the tank; dithiocarbamates such as mancozeb and metalaxyl are both residue-sensitive in export grapes.
- Never exceed label dose; a doubled dose risks phytotoxicity and rejection of the consignment.
- Keep a spray record (product, dose, date, PHI) and give the buyer the interval in writing.
- Also remove the most heavily infected leaves tomorrow and open the bunch zone - a fungicide cannot compensate for a canopy that stays wet.
3. List the cultural and monitoring measures you would put in place for the rest of the season, and explain how you would use them to test whether resistance was really involved.
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Cultural measures
- Shoot thinning and positioning; remove 3–4 basal leaves around the bunch zone so it dries quickly.
- Avoid excess nitrogen, which produces dense, soft growth that holds humidity.
- Keep the floor mown and dry; drip in the morning, never overhead and never in the evening.
- Remove and destroy infected leaves and shoot tips rather than leaving them in the inter-row.
Monitoring plan
- Scout twice a week in humid weather, checking the undersides of leaves in the lower and middle canopy.
- Use a simple weather rule: 10 mm or more of rain with 10+ hours of leaf wetness at 10–24°C is an infection period. Spray the protectant before the next such forecast, using a leaf-wetness sensor or the local agromet advisory where available.
- Keep a spray diary with date, product, FRAC group and dose, and enforce a rule that no FRAC group is used more than twice in a season and never in consecutive sprays.
- In low-pressure periods, protectants or a biocontrol such as Bacillus subtilis can replace the systemic.
Testing the resistance question
A resistance claim is only proven if a new mode of action with good coverage actually stops new spots. If new oil spots continue after a correctly timed, correctly applied spray from a different FRAC group, look for application or coverage failure before blaming resistance. Reviewing the spray diary at the end of the season - which sprays actually halted new lesions - is the practical test.