Tomatoes: With all the rain, common fungal diseases on tomatoes are starting to show up. Two of the most prevalent ones, Septoria leafspot and Early Blight, are soil-borne diseases that build up in the soil when tomatoes or tomato family plants are grown in the same place every year. Solonaceae, the tomato family, also includes potatoes, peppers, tomatillos and eggplant for commonly grown vegetable plants.
Septoria leaf spot symptoms begin on foliage nearest to the ground and move upward. Leaf spots are generally small and circular with dark borders and gray or tan centers. The spots eventually coalesce into larger lesions. Warm, wet, humid weather increases the severity of the disease to the point that all leaves die and drop. Go to pddc.wisc.edu/ and under the fact sheet tab, search for “Septoria” for more information.
Early blight appears as brown to black lesions with concentric rings in the center on the top side of the leaves. Like Septoria, lesions usually first appear on older, lower leaves and progress upward. Lesions may be accompanied by yellowing. Warm, wet, humid weather favors early blight development along with alternating periods of wet and dry weather that tend to increase disease severity. Visit pddc.wisc.edu/ and search for “Early Blight” under the fact sheets tab for more on this subject.
Some things you can do to protect tomatoes from these two foliar diseases are trellising, pruning and mulching. Trellising tomatoes keeps them off the ground, so they are not in direct contact with soil that carries the spores.
Keep the plants thinned out for good air circulation by pruning off suckers and selectively removing crowded stems or leaves. Remove lower leaves close to the soil to avoid spores of the early blight or Septoria leaf spot fungus from splashing up on them during irrigation or rain. Don’t overhead-water tomatoes (this also keeps leaves dry); water them at the base of the plant.
Mulching with clean straw or hay or even a few sheets of newspaper (not colored shiny sheets, though) will also help control the initiation and spread of these diseases by creating a barrier between the spores in the soil and the plant leaves.