Tuesday, November 9, 2010
The last few days have been a frenzy of black spots on my young tomato plants. But before I get to methods to control these, it's worth trying to figure out what they are exactly.
There are a number of organisms that can cause black spotting on tomato leaves, especially when the plant first sets fruit. They include Septoria leaf spot (a fungus), bacterial spot (Xanthomonas campestris), bacterial speck, and few others. In general, these are controllable, while some of the more terrifying wilts and blights are killers.
Most of these conditions are caused by the rapid spread of an infectious organism among your tomato plants. Just like in people, these little buggers like warm, wet conditions, so the early part of the season is especially dangerous for us as it's typically still hot and muggy—perfect weather for a bacterial or fungal infection.
In this case, I'm guessing I have Septoria leaf spot, which is characterized by wet lesions on the underside of leaves that develop in black spots with gray centers. Septoria is a fungal infection caused by warm, wet conditions. It is pretty much always present in South Florida, so ... you know, hard to avoid.
I always hate seeing this happen, even though it happens every year.
To control these kinds of diseases, start with prevention. Plant tomatoes in fresh soil, leave them far enough apart that the organism can't easily jump from plant to plant, and never water from overhead.
After you get it, this is what I do: apply a copper fungicide spray according to the label directions (weekly) and religiously remove any affected leaves. Copper fungicide is rated for organic growth, so you can use it on your plants and still have organic tomatoes. So obviously, it's not the strongest fungicide on the market—if you really want to go ninja on fungal diseases, you can switch to a much stronger product such as maneb or mancozeb. These are commercial-strength products.
Personally, I don't use 'em. I cut away leaves, slow it down with the copper spray, and hope the temperatures and humidity break in time for the plant to outgrow the infection. This has worked for me well enough, and I can avoid the stronger fungicides.
As a side note, I'm still really looking forward to the yellow pear tomatoes. EVERYONE loves them. But, man, me and yellow tomatoes have a rocky relationship, and this year is no different. These things are driving me crazy—leaf roll, fungal diseases, and of course, the tomato leaf curl virus. If anyone out there has experience successfully growing yellow tomatoes in South Florida, I'd love to pick your brain ...