Plant Doctor: Tomatoes in distress!

While admiring the tomatoes during our Dillo Day Recovery Gardening last Sunday, I noticed that the tomatoes had some strange-looking spots on their leaves (see pictures below).  The leaves had lots of white/gray splotches interspersed with black dots.  The spots were most dense on the bottom (oldest) leaves of the plants, while the new leaves seemed relatively unaffected.

In the past year, tomatoes have gotten a lot of press for their fungal diseases, which can reduce yields due to leaf damage or even infect the fruit itself.  Last summer, a late blight epidemic swept across the Northeastern US due to wet, cool conditions and due to an increased demand for home-grown veggies because of the Recession.  What happened was this: a few major plant distributors (like Home Depot, Kmart, Lowe’s and Wal-Mart) purchased starter plants from the Southeast and transported them north.  Along the way or at their point of origin, these starter plants got infected with late blight fungal spores, which, once the plants were in the ground, swept through home gardens and windowsills, under the radar of disease specialists, and devastated home gardens and commercial farms alike.  To a fungus, which can disperse spores up to 40 miles, home gardens of susceptible tomatoes were like one large farm: or one large meal.  For more information about the 2009 late blight in the Northeast, check out the NY Times article here: http://www.nytimes.com/2009/08/09/opinion/09barber.html

Late blight is notorious: a strain of this fungus caused the Irish Potato famine.  But other diseases affect tomatoes too.  Here are some examples of the most common diseases and how to identify them so that you can look for them in your own garden.  I’m only including foliage diseases, since we don’t have to worry about diseases that affect the fruits at this point in the season:

Note:  You can reduce all tomato diseases by limiting moisture on foliage (so use drip irrigation or try to just water the soil with your hose).  Crop rotations of at least 3-year cycles are also essential.  When rotating crops, keep in mind that peppers, eggplants, and potatoes are also in the family Solanaceae and thus can be carriers of fungal diseases that affect tomatoes.

Early Blight:  Caused by the fungus Alternaria solari.  Identified by characteristic dark spots with concentric rings which develop on older leaves first.  The surrounding leaf area may turn yellow.  Since this fungus is soil-bourne, it is difficult to eradicate.  Treatment involves removing affected plants, staking and mulching in order to keep foliage and fruits off the ground, and the use of organic fungicides.  There are no tomato varieties resistant to early blight.

Gray Leaf Spot: Caused by the fungus Cercospora zeae-maydis. Identified by small, dark spots that can be seen on both the top and bottom surfaces of the leaves. The spots enlarge and turn a grayish brown. Eventually the centers of the spots crack and fall out. Surrounding leaf areas will turn yellow and the leaves will dry and drop.  Treatment involves removing infected leaves and plants and planting resistant varieties.  This is what I believe our tomatoes at Wild Roots are infected with, so we’ll keep removing foliage and attempting to keep water off the foliage to reduce the impact of this fungus!

Late Blight: You guys already read about this one.  Symptoms are greasy looking, irregularly shaped gray spots appear on leaves. A ring of white mold can develop around the spots, especially in wet weather. The spots eventually turn dry and papery. Blackened areas may appear on the stems. The fruit also develop large, irregularly shaped, greasy gray spots. Treatment involves copper sprays, removing infected plants, and planting resistant varieties.  If you have light blight, you should consider not saving seed potatoes from your garden, since the fungus can overwinter there!

Verticillium Wilt: Verticillium wilt is caused by a soil-borne fungus and it can affect many different vegetables.  Symptoms include wilting during the hottest part of the day and recovering at night, yellowing and eventually browning between the leaf veins starting with the older, lower leaves and discoloration inside the stems.   Identify this one by cutting into the tomato stem just above ground level; if you have verticillium wilt, you’ll see that the transport tissue in the stem is brown.  Because this fungus is soil-borne, it is difficult to eradicate once you’ve got it, so treatment involves removing infected individuals and planting resistant varieties.

Septoria Leaf Spot: Caused by the fungus Septoria lycopersici.  Septoria Leaf Spot is sometimes mistaken for late blight.  Symptoms include papery patches on the leaves which develop tiny, dark specks inside them. Older leaves are affected first.  In the center of the spots are many dark brown, pimple-like structures called pycnidia-fruiting bodies of the fungus; you can see these with the naked eye or with a hand lens; these fruiting bodies distinguish septoria from late blight.  Treatment involves removing infected foliage.  Since this disease is often seed-borne, buy seed from reputable sources and do not save seed from infected plants.

Tomato diseases are scary, but if you catch them and identify them early, rotate crops, and reduce water contact with foliage, you can still harvest a bountiful crop. I hope that your gardens are healthy and productive!

– Jackie B.

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