When should I start preventive spraying for tomato blight?

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Published: October 23, 2025

Updated: October 23, 2025

Start preventive spraying for tomato blight two weeks after transplanting or when humidity exceeds 80%. These are the critical thresholds that produce spore germination. I start taking humidity readings with simple hygrometers placed among the plants. Apply copper fungicides before rainy weather, since wet weather hastens the spread of blight. Reapply every 7-10 days during long wet periods for continuous protection.

Preventive Spraying Schedule by Risk Level
Risk Indicator	First Application	Frequency	Product Type
Low (Humidity <70%)	3 weeks post-transplant	Every 14 days	Milk solution or baking soda spray
Moderate (Humidity 70-80%)	2 weeks post-transplant	Every 10 days	Copper fungicide
High (Humidity >80% or rain forecast)	Immediately	Every 5-7 days	Copper + mancozeb rotation
Extreme (History of blight)	At transplanting	Every 7 days	Chlorothalonil rotation

Always spray early morning below 85°F (29°C)

Preventive Spraying Schedule by Risk Level

Risk Indicator

Low (Humidity <70%)

First Application3 weeks post-transplantFrequencyEvery 14 daysProduct TypeMilk solution or baking soda spray

Risk Indicator

Moderate (Humidity 70-80%)

First Application2 weeks post-transplantFrequencyEvery 10 daysProduct TypeCopper fungicide

Risk Indicator

High (Humidity >80% or rain forecast)

First ApplicationImmediatelyFrequencyEvery 5-7 daysProduct TypeCopper + mancozeb rotation

Risk Indicator

Extreme (History of blight)

First ApplicationAt transplantingFrequencyEvery 7 daysProduct TypeChlorothalonil rotation

Always spray early morning below 85°F (29°C)

Monitor plants daily during peak season for environmental triggers. Humidity higher than 80% for 48 hours requires immediate spraying. If there has been a period of heavy rain, then a spray of fungicides needs to be reapplied within 24 hours. Weather apps that have humidity alerts can be beneficial for rapid notification. I keep a sprayer pre-mixed with sprays during hazardous periods, where rapid notification can result in immediate oil or fungicide spraying.

Product Rotation

Alternate chemical families monthly
Combine organic and synthetic options
Never apply same fungicide consecutively

Application Technique

Cover both leaf surfaces until runoff
Use fine mist sprayer at 40 psi pressure
Apply during calm morning hours
Add spreader stickers for rainy seasons

Integration with Culture

Coordinate with mulching schedules
Time pruning before applications
Align with drip irrigation cycles

For top results, it is best to combine spraying with cultural practices. Space the plants 24 to 36 inches apart so that the spray may enter through the foliage. Use drip irrigation to keep the foliage dry. Apply straw mulch before the first spray to prevent soil from splashing upon the plants. The above practices have increased the efficiency of fungicides by 40% in my garden tests.

Alter your spraying calendar by region. Gardeners along the coast had been spraying earlier due to the marine layers. The south needed more summer sprays to control the insects, which were more populous. Keep a careful record of the pattern of infection in this locality, and the application can be better timed. My spraying records have shown that starting 10 days earlier in regions where humidity causes the insects to breed abundantly and can be properly timed to prevent initial infection.

Read the full article: Tomato Blight Treatment Guide: Control & Prevention

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