I found dark, splash-spotted lower leaves on the raised tomato beds the morning after a warm rain. I crouched down and saw the same speckling on a dozen plants. Those spots meant the spores had landed and gone to work, and the protective window for that flush of growth had closed the day before. The right anthracnose spray timing puts the fungicide on healthy leaves before the wet spell, not after the damage shows up. You spray before rain, while the tissue is still clean.
This trips up a lot of gardeners, and bad anthracnose spray timing is the usual reason. The plant looks fine, so you wait. But anthracnose spreads when spores splash onto wet leaves and sit there long enough to take hold. By the time you see the lesions, the infection cycle is already done for that round.
Here is why the timing matters so much. Common garden fungicides are protectants, which means they coat healthy tissue and stop new spores from getting a foothold. They do not cure a leaf that is already infected. Spray a spotted leaf and you waste the product, because there is no fixing damage that has already started. The whole point is to have that coating in place before the next wet spell hits.
Watch two things: the forecast and the leaves themselves. Spores need leaves to stay wet for about four hours to infect, so a quick shower that dries fast is low risk. A warm, humid stretch with morning dew that lingers is high risk. When you see a wet spell coming for susceptible plants, get your protective spray on a day or two ahead of it.
Cool, wet spring weather is the danger zone, roughly 50 to 68°F (10 to 20°C) with high humidity. That is when spores are most active and leaves stay damp longest. Your fungicide schedule tightens up in this window. In cool, wet weather you reapply protectants at the shorter 7 to 10 day interval. Rain and fresh growth keep stripping away your protective layer. Once the weather turns dry and settled, you can pause. Infection risk drops off a cliff without leaf wetness.
Before bud break or planting
Choose resistant varieties and clean up last year's infected debris so fewer spores are waiting.
Early, cool wet spring
Begin protectant sprays on susceptible plants before predicted wet spells, not after lesions appear.
During wet, humid weather
Reapply protectants at the shorter 7 to 10 day interval while leaves keep getting wet.
Dry, settled weather
Pause spraying once conditions dry out, since infection risk drops sharply without leaf wetness.
Big established trees are a different story. A shade tree that drops its spotted leaves in a cool spring almost always pushes out a healthy second flush by midsummer. The tree shrugs it off. You rarely need to spray a mature tree at all. Save the spray for a tree that loses its leaves several years in a row. Even then the spray has to coat the new buds in spring, and that is hard to reach on anything tall. I leave my oaks alone and they bounce back fine each year.
So skip the urge to chase the disease once you see spots. I pull and toss the worst infected leaves to cut down spore numbers, then aim my real effort at the next clean flush of growth. Put the protectant on before the next wet stretch. Keep that coating fresh on the 7 to 10 day rhythm while the leaves stay damp. Do that and you stop the cycle where it actually starts, instead of paying for a spray that lands too late to help.
Read the full article: Anthracnose Disease: Identify and Stop It
