I get phone calls from gardeners all the time, and this year I have received a lot concerning tomatoes. Many seem to be experiencing problems on their tomato plants so here’s an update on some of the diseases.
The most serious is late blight which is caused by a fungus, develops very quickly and invariably destroys the crop. Greyish-looking, irregularly shaped patches appear on the leaves sometimes accompanied by a white mould. The grey spots become dry and papery and black areas will develop on the stems. There is no cure for this disease; prevention involves a regular spray program using a copper-based fungicide before it strikes.
This disease organism does not usually live over winter in our cold climate and the fungus comes in on the wind. My area had a severe outbreak a few years ago when most gardeners lost their tomato crop. Even if picked when the first signs of the disease are noticed, the fruit will rot before it ripens. This serious disease was the cause of the historic potato famine in Ireland.
Early blight, although serious, is not as difficult to deal with as late blight. Early blight, also caused by a fungus, can live over winter in the soil. Practising good hygiene by cleaning every scrap of tomato foliage off the garden in the fall is important in preventing the development. A copper-based fungicide will prevent the disease and the application of such a fungicide will halt the development of it if discovered early enough. The symptoms of early blight include dark spots on the leaves with concentric rings around them. The leaves turn yellow and then die — older leaves are the first to succumb.
Another fungal disease, septoria leaf spot, causes paper-like patches to develop on the leaves and tiny dark specks appear on the patches. Older leaves are the first to develop the symptoms; a fungicide spray program will control the disease.
There are two wilts that attack tomato plants and these diseases are often mistaken for the dreaded blights. Fusarium wilt and verticillium wilt both cause the leaves to turn yellow and then brown. The plants may appear wilted during the day but recover at night. These diseases most often develop during cool weather. Eventually the entire plant will die although the fruit, if picked early enough, is often usable.
The best preventive measure to take against wilt diseases is to use resistant varieties. The capital letters V and F after the variety name on the plant tag or in the seed catalogue will indicate a resistance to the diseases.
Two other afflictions that affect tomatoes are caused by wide fluctuations in moisture levels during the growing season. One of them is blossom end rot. This pathogen causes the fruit to develop rot at the blossom end and eventually the fruit rots. Too much nitrogen in the soil will exacerbate the problem; therefore it is not wise to use a high-nitrogen fertilizer on tomatoes. The disease is encouraged by a lack of calcium in the soil so adding bone meal or crushed eggshells to the planting holes at planting-out time will help to ward off the disease. Mulching the plants will help to maintain a more even moisture level in the soil.
The other disease, called fruit cracking, is also the result of rapid growth and too much fluctuation in moisture levels. The cracks can be radial, radiating out from the stem on the top of the fruit or develop in concentric circles around the stem. Although the cracks are unsightly, they do not make the fruit inedible and the damaged portions can be cut off and the fruit used as usual.
Catfacing, another form of cracking, occurs at the blossom end and may involve bulges and deformation as well as cracks. Cold weather at the time of blossoming can result in some tissue death that produces misshapen fruits with lots of bulges and crevices. The symptoms are usually only on those fruits that developed at a particular time and most will be unaffected; the disease does not attack the foliage and the plants will look healthy — which they are.
Many tomato diseases are weather related — too much moisture, too cool temperatures, long dry spells followed by heavy rainfall are examples. The more serious fungal diseases will develop more often in wet weather and vigilance will be required to combat these.
Tomatoes are so popular they are included in almost every garden. If you grow tomatoes, you will need to know how to protect your plants from some of the more prevalent diseases so that you can reap the rewards of your efforts.