Tomato Late Blight

Let me first start off by saying Late Blight of Tomato HAS NOT BEEN FOUND IN WNC, yet. But, it has been confirmed very close by in Northern Georgia.

Late blight of tomato is caused by Phytophthora infestans, the same pathogen that wiped out the potatoes in the Irish Potato Famine of the 1840s. P. infestans is an oomycete organism, a group that is also known as the water molds. The pathogen enjoys cool, wet weather conditions. P. infestans spreads and kills a plant very quickly, especially in ideal conditions. Spores of the pathogen are easily disseminated from infected areas to other areas via wind currents, these spores are protected by cloudy conditions.

Symptoms of late blight appear as large, olive green to brown spots on leaves. You may see a fuzzy, white fungal growth on the underside of the leaves. Symptoms also appear as brown or black lesions on the stems and fruit.

To see some excellent pictures of late blight, visit Cornell's Long Island Horticulture and Extension Center's site and the NCSU Factsheet on Tomato Late Blight.

Late Blight is often confused with Early Blight. Unfortunately, these diseases have similar names, but the symptomology and causal organisms couldn't be more different. If you see bulls-eye lesions on leaves starting on the bottom leaves, you are most likely dealing with Early Blight, a fungal disease, caused by Alternaria solani. Revisit my post Early Blight of Tomatoes for pictures and additional information. Compare symptoms of Early Blight with the pictures of Late Blight from Cornell's link above. Cheryl Smith at UNH has also put together an excellent resource comparing the 2 diseases.

If you think you have Late Blight of Tomato, please contact your local extension office to have it positively diagnosed.

Late blight has been all over the news because of its early arrival in the Northeastern US, which has been experiencing cool and wet weather as of late. The disease doesn't usually occur in that region until late-July or August. This year it is believed that the pathogen hitch-hiked on transplants to retail stores and then to home gardens. It is believed that the disease appeared as early as mid-June.

To read more about the problem in the Northeast visit the following sites/article:

UMass Extension "Late Blight Alert for Potatoes and Tomatoes"

American Vegetable Grower "Late Blight on Potatoes, Tomatoes Confirmed in Northeast" - contains information on management as well

Providence Journal "Irish Potato Famine Fungus Strain Hits R.I. Crops"

Early Blight of Tomatoes

One of the first disease problems I have encountered in WNC is Early Blight on tomatoes. Early blight is caused by Alternaria solani, a fungal pathogen. Traditionally, symptoms of Early Blight appear as spots on the foliage. These spots have concentric circles and look like a "bull's eye" (image, right- click on image to enlarge). These symptoms first appear on older leaves of the tomato plant.

In addition, Early Blight can actually affect the stems of the tomato plants (image, below). This condition is known as "collar rot". Notice that the stem lesion begins where the plastic begins, not at the soil line. Concentric circles may also be seen in the stem lesions. Early Blight attacking stems can be an severe problem, especially in fields that are not fumigated, as plants will not be able to survive stem girdling.

In order to control A. solani and other important pathogens of tomato, prevention is key. Dr. Kelly Ivors and Dr. Frank Louws, both of the Department of Plant Pathology at NCSU have put together a comprehensive Foliar Fungicide Spray Guide for Tomatoes in NC. This is program is a holistic approach to tomato disease control and followed by many WNC tomato growers.

Other fungicides that are very effective for Early Blight on tomato are the strobilurin fungicides, azoxystrobin (Amistar, Quadris) and pyraclostrobin (Cabrio). *Strobilurin fungicides must be rotated with a fungicide that is not in FRAC group 11.* Other fungicides with good efficacy towards Early Blight are boscalid (Endura), mancozeb (Dithane, Manzate, Penncozeb, Manex II) and maneb (Manex, Maneb).
Efficacy data obtained from Southeastern Vegetable Extension Workers 2008 Vegetable Crop Handbook. Always follow application guidelines on the fungicide label!