Late Blight And Downy Mildew

 

 

The recent wet weather has promoted many of these diseases. For several years, we have been talking a lot about two of them; late blight and downy mildew. Both of these diseases are in the phytophthora family of disease. You could say they are first cousins. Late blight can destroy a healthy tomato or potato plant in a matter of a few days.  It does not attack other vegetable or fruiting plants. Late blight is a fungus that moves through the air. It can move long distances when the weather is cool and wet. You will first notice a dark colored blotch with a light green hallow around it on an upper leaf. When you look on the underside of the leaf, the lesion will have a light grayish colored, fuzzy ring around the outside. This ring of fuzzy growth is the formation of spores that will be released into the air to infect additional plants. 

 

Because late blight makes thousands of spores that can travel long distances, it is critical that you watch your tomato and potato plants carefully. If you think you have late blight, please let your county Penn State Extension office know as soon as possible. A positive identification will be needed. Also, other gardeners and farmers need to know if late blight is in their area.

 

Downy mildew is also deadly and can take out a cucumber plant in about a week. It too moves great distances through the air. It infects cucumber, cantaloupe, watermelon, squash (summer and winter), and pumpkin. Downy mildew also infects grapes and impatiens.

 

Fungicides are effective at preventing late blight and downy mildew if they are applied before the spores land on the leaf. Phytophthora diseases can’t be eradicated once they have infected a plant. Fungicides containing Chlorothalonil are the most effective at preventing late blight and downy mildew. Fungicides containing Mancozeb are second best. Copper fungicide is somewhat effective and some formulations of copper fungicides are approved for organic production.