Fungicides and Rain

Many vegetable and berry growers across Pennsylvania have experienced frequent rainfall this season. The week ending on August 24, the average rainfall amount in the state was 1.5 inches. The high was 4.11 inches in Erie County and the northern tier and eastern portion of the state had the lowest totals recorded for the week.  While harvest is winding down for some crops, many fields still contain marketable produce that needs protection from foliar diseases.

When rain is in the forecast, one of the most common questions is: Should I spray before the rain or after? The short answer is usually before the rain. However, the decision is not always straightforward. The effectiveness of a fungicide application depends on several factors, including the product used, the amount of expected rainfall, disease pressure, and application timing.

Fungicide Decision Guidelines

Here are a few guidelines to keep in mind when making fungicide decisions during wet weather:

Proactive Sprays are Best

Apply fungicides preventively or at the first sign of disease, when infection levels are still low. Most fungicides provide little to no curative activity once disease is established. Waiting to spray until symptoms are widespread results in poor control and crop losses.

Spray on Dry Foliage

Whenever possible, apply fungicides when leaves are dry. Dew or moisture on plant surfaces can dilute spray droplets and increase runoff, reducing coverage and effectiveness.

Allow Time for the Spray to Dry

Most fungicides need time to dry and bind to the plant surface before rainfall occurs. A common rule of thumb is to allow at least four hours of drying time, although this can vary depending on the product and weather conditions. Always consult the product label for guidance.

Consider Adjuvants When Appropriate

If the fungicide label recommends an adjuvant, use one. Adjuvants improve coverage, sticking ability, or penetration. Sticker-spreader products can help fungicides remain on the plant surface during rain events.

Frequent Rain May Require Shorter Spray Intervals

Periods of repeated rainfall can reduce fungicide protection in two ways. First, rain can remove fungicide residues. Second, rapid crop growth produces new leaves and shoots that are unprotected. In these situations, shorter spray intervals may be needed to maintain protection. Be sure to keep track of annual maximum application rates listed on the label.

Diversify Your Fungicide Program

Do not rely solely on protectant fungicides such as captan, thiram, mancozeb, or chlorothalonil. Including effective single-site, systemic fungicides with activity against your target disease can give better coverage and improve rainfastness.

What Does Rain Do to Fungicide Residues?

Rainfall can remove fungicide residues, but the relationship is not always simple. Research has shown that some fungicides are more vulnerable to wash-off than others and that the amount of residue lost depends on rainfall intensity and timing.

In a Michigan State study, about half of the captan residue on apple leaves was removed by as little as 1 millimeter (less than 1/10 inch) of rain. However, once this portion had washed off, additional rain caused much less residue loss. Other studies in grapes and blueberries found that 1/10 inch of rain may remove 20 to 25 percent of protectant fungicides such as ziram and mancozeb, but it takes 1 to 2 inches of rainfall before disease control is noticeably reduced.

The age of the spray residue also matters. Residues a week old that have been degraded by sunlight and microbial activity are more vulnerable to wash-off. In some cases, only about 0.5 inch of rain may reduce older residues below effective levels.

A useful rule of thumb is that 1 inch of rain may remove about half of the fungicide residue, while 2 inches or more can remove most of the remaining protection. Heavy downpours generally remove more fungicide than several lighter rain events.

The Bottom Line

When rain is in the forecast, a timely preventative fungicide application is usually a better option than waiting until after the storm. Apply to dry foliage, allow adequate drying time, and consider shorter spray intervals during frequent rainy spells. After significant rainfall or rapid plant growth, reevaluate and determine whether another application is warranted. A well-timed spray before the rain often provides better protection and a better return on investment than trying to manage disease after it becomes established.