Marge in Ackley has some yellowing on her tomato plants along with some brown spots. It could be a couple of things, but it is probably Septoria Leaf Spot. This is the most common leaf disease of tomatoes in Iowa.
It is caused by the fungus Septoria lycopersici and first appears as small spots that soon become about 1/8 inch in diameter. These spots gradually develop grayish white centers with dark edges. The light-colored centers of the spots are the most distinctive symptom of Septoria leaf spot. Spores are spread to new leaves by splashing rain and heavily infected leaves will turn yellow, wither and eventually fall off. The lower leaves are infected first and the disease will progress upward if the rainy weather lasts awhile. And we have had plenty of that.
The fungus survives the winter in tomato debris so it is important to clean up the bed after the growing season is finished.
The control of Septoria leaf spot is a combination of cultural practices…. and they can also help reduce the risk of many other diseases. Here they are (from the Iowa State University Extension service):
-Plant transplants far enough apart that the plants will not be crowded after they are full grown in order to help the leaves dry out rapidly.
-Water at the base of the plant to minimize the amount of time the leaves are wet. And water in the morning rather than the evening.
-Remove as much debris as possible in the fall.
-Rotate crops so that tomatoes are only grown in the same ground every three or four years.
-Avoid working with the plants when the foliage is wet to avoid spreading the disease-causing microorganisms.
One of the other ‘yellow-leaf-black-spot’ diseases on tomatoes is Early blight. This is caused by the fungus Alternaria solaniand can also be called Alternaria leaf spot or target spot.
This is also common on Iowa tomatoes and can attack the same plants that have Septoria leaf spot. Losing the lower leaves early is the most obvious symptom of this disease.
The fungus can sometimes attack fruit at the stem end, causing large sunken areas with concentric rings and a black, velvety appearance.
Warm and wet weather (sound familiar?) favors the rapid spread of early blight.
You can use the same procedures to control early bite as those for Septoria leaf spot. Also avoid rotating with potatoes.
Next up is Bacterial spot. This is caused by a bacteria, Xanthomonas campestris pv. vesicatoria and infects both tomato and pepper plants.
Warm and rainy weather favors the rapid spread of bacterial spot.
Use the same control measures as for Septoria leaf spot, although, buying disease-free transplants is particularly crucial for controlling this and other bacterial diseases, since the bacteria can be transmitted to the seedlings from contaminated seeds. Avoid rotating with peppers and avoid handling infected plants any more than is necessary.
Bacterial speck is caused by the bacterium Pseudomonas syringae pv. tomato and does not affect peppers but may survive on non-host plants
The specks are a lot smaller than the spots caused by bacterial spot, don’t go into the fruit too deeply and can be scraped off with a fingernail. It usually doesn’t reduce yields greatly, but it can affect fruit quality. This type of infection is favored by cool, wet conditions. Control is pretty much the same story: follow the same measures as for Septoria leaf spot.
So essentially, make sure there is plenty of air flow around your tomato plants, water in the morning and at the base of the plant, clean up any debris from the bed and rotate, rotate, rotate!
There are a lot of other things that can affect one of our favorite summer fruits. To read up on more (and where I got the above information from) about Tomato diseases and disorders click here.
And good luck with your tomatoes!
Posted under Diseases
This post was written by Eileen Loan on June 28, 2010