Watch Out For Diseases!
Since our dry spell was broken last week with some damp weather, followed by quite a bit of rain, and now humidity, we are seeing great weather for disease problems!
The two diseases that I will be keeping a lookout for in the Demo Garden are Black Rot on the grapes and Septoria Leaf Spot/Early Blight on the tomatoes.
I have already seen some indications of the Black Rot on the grapevines. Last week I saw this leaf with the characteristic lesions. This morning I am seeing the first of the green grapes with the spots on them. Last year I think we lost half of the grapes to this fungus. The best way to prevent the disease is to spray with either a copper-containing fungicide or myclobutanil (Immunox, but not Immunox Plus!). The fungicides should be sprayed every 7-10 days when the conditions are right for infection. At temperatures between 70 and 85, 6-9 hours of continuous leaf wetness will result in infection. As the temperature climbs to 90 or hotter, it takes 12 hours of leaf wetness for infection to occur.
Septoria Leaf Spot and Early Blight are two extremely common tomato diseases in this area. We haven’t seen any sign of them yet, but the symptoms and signs will probably start showing up after this warm, moist spell.
This tomato leaf shows classic symptoms of Septoria. If you click on the picture, you can clearly see the tiny black spots all over the leaves. The spots generally appear on the lowest leaves first, causing the leaves to turn yellow and die. The disease moves up the plant as the summer progresses. Early Blight is very similar, except that instead of small black spots, you see 1/2″ lesions with concentric circles (bull’s eye) on the leaves.
There are numerous cultural controls for this diseases, including removing diseased leaves, using clean straw or other mulch around the plants, using cages/other methods to keep plants off the ground, and spacing plants to allow for good air movement.
Applying a copper-containing fungicide or chlorothalonil will also help prevent disease problems.