Preventing Foliar Tomato Blights

Published: May 1, 2011

One of the most revered summer vegetables by home gardeners is the tomato. Wet growing seasons aggravate our most common foliar diseases, often referred to as blights. All of Missouri was very wet in 2008 & 2009; most of the Central, Eastern and Northern parts of Missouri were also wet in 2010. These diseases are caused by two fungi, Septoria leaf spot and Early Blight, and two bacteria, Bacterial Spot and Speck. The disease descriptions are presented below, and one web site link provides color pictures, to aid one in trying to diagnose a foliar problem they may have experienced. However, correct diagnosis can be extremely challenging, especially when two or more diseases are occurring at the same time, as has often been the case in the last three years. It is best to have plant problems either diagnosed by a local extension agent or submitting the sample to the Plant Diagnostic Clinic. Please note that bacterial diseases are controlled with different chemicals than fungal diseases, as the biology of the diseases is different. Many gardeners have been applying a fungicide when they have had a bacterial disease, which will not provide any control. However, the most serious problem has been inattention to disease prevention, which begins at planting, and is the focus of the remainder of this article.

Preventing foliar diseases should start by minimizing disease inoculum, which can be harbored on diseased plant residue or wooden stakes in contact with it. Dispose of tomato residue away from the vegetable garden, or burn it. If tomato diseases have been a problem, consider changing to metal stakes/support or sterilize wood stakes by soaking in a bleach solution. Proper spacing is also crucial to preventing disease. It is better to give a little more space than is required than crowding plants. Over-crowding plants encourages disease by increasing the humidity, decreasing air movement, and increasing competition between the plants. Several common foliar diseases exist in the soil and are spread to the plant by splashing water from irrigation and rain. Mulching around plants will help by reducing water-splashed soil and the pathogens that are spread with the particles. . However, organic mulches like straw will suppress soil temperatures and may keep the soil too wet during periods of heavy rainfall in the spring. This can aggravate a common root/stem rot- Fusarium wilt (see below). Commercial growers use plastic mulch (usually black) to warm the soil and prevent splashing of soil particles, but home gardeners are often reluctant to use plastic because it requires irrigation under the plastic. Black weed barrier cloth, 'landscaping fabric' or a similar product is an ideal choice of material as it allows water to pass through, while still providing the benefits of plastic mulch.

Many gardeners wait until they see disease symptoms before they apply a chemical for disease control. This is later than desired, as 'preventing' the disease is easier than controlling it. Applying a chemical control is best immediately after weather conditions favorable to foliar tomato blights. These conditions are spells of several days of rainy, warm and humid weather, which typically begin in late May to early June. Thus a good 'plan' is chemical control applied about the 3rd week of May and again two weeks later (1st part of June), while being willing to adjust based on the weather conditions. After a couple of preventative chemical control applications, one can wait to see disease symptoms before continuing with chemical control.

Chemical control by home gardeners of the bacterial diseases is primarily with copper products, which are available in formulations that are approved as organic products. In the last couple of years an organic fungicide has become available to home gardeners which is labeled 'for control or suppression' of all the foliar tomato blights, bacterial and fungal. Serenade Garden by AgraQuest has an active ingredient of QST 713 strain of Bacillus subtilis, a beneficial bacterium. Home gardeners should also consider including a product into their spray solution that enhances coverage and adherence to plant tissues. A number of companies manufacture and sell 'spreader/sticker' which help the solution to spread evenly out on surfaces instead of 'beading up' and rolling off. Once the solution has dried, its adherence to a surface is improved when rainfall occurs. Given these two benefits, the addition of a 'spreader/ sticker' to a spray solution will improve the level of control.

An important cultural control method often overlooked is vigorous growth- dark green foliage typical of a plant with adequate nutrition grown in full sun. Tomatoes in a somewhat shady location (e.g. 3 hours or more per day) will be more disease prone. Nitrogen (N) is the element most often limiting to plant growth, and tomatoes have the highest N requirement of all vegetables. Side dress recommendations are one pound of actual N per 100 linear feet of row at three times- one to two weeks before the first fruit ripens, two weeks after picking the first ripe fruit and one month later. Nitrogen can be supplied from natural or synthetic sources, with the latter easier to estimate and apply, as well as more quickly available to the plant. For detailed information about fertilizing vegetables see the MU Guide 'Steps in Fertilizing Garden Soil: Annual Flowers and Vegetables- http://extension.missouri.edu/explorepdf/agguides/hort/g06950.pdf

• Anthracnose (fungus): Anthracnose fruit rot was prevalent throughout the Midwest last year, aggravated by the very warm and humid summer conditions. It is spread in an odd way, establishing in the leaf lesions of Early Blight and then splashed onto the fruit. The chemical controls for Early Blight also control it. Infections begin on green fruit and, symptoms become most apparent when ripe. Small, water-soaked, slightly sunken circular spots on fruit (sometimes called 'ripe rot'). Cultural controls consist of sanitation*, crop rotation, weed control, proper plant spacing and staking, and watering at the soil level. Chemical control includes applications of chlorothalonil, maneb, or mancozeb.
• Bacterial spot (bacterium): small, angular, water-soaked spots on leaves and stems. Raised, crusty spots on fruit. Cultural controls include planting healthy transplants, sanitation*, crop rotation, using metal stakes (or treat wooden stakes) and watering at the soil level. Chemical controls include copper hydroxide or similar copper based products.
• Bacterial speck (bacterium): The foliar symptoms of speck consist of small (1/8-1/4 in.) black lesions, often with a discrete yellow halo. The lesions of bacterial spot are similar, but tend to have a greasy appearance, whereas those of speck do not. Speck seems to curl the leaves more severely than spot. Both diseases affect flowers. Lesions on stems and petioles cannot be distinguished. Bacterial speck and spot are more clearly differentiated by symptom development on the fruit. Bacterial speck lesions are slightly raised, but are generally much smaller (1/16 in.) than those of bacterial spot. Bacterial speck lesions are very superficial and do not crack or become scaly as in bacterial spot. Control methods are the same for Bacterial spot.
• Early blight (fungus): starts at bottom of plant and advances upwards, as dark-brown circular spots with concentric rings, or "targets," on leaves. Tissues around spots become yellow. When spots are numerous, leaves wither and dry up. Plant healthy transplants; other controls are the same as for anthracnose.
• Fusarium wilt (fungus): lower leaves turn yellow and dry. Leaves roll up and wilt during hot part of day. Inner stem tissues have dark discoloration. Cultural control includes planting healthy, disease-resistant** transplants, sanitation*, and crop rotation. No chemical controls are available. Disease resistance traits have been bred into many tomato varieties. These varieties are much less susceptible to the diseases than they do not have a resistance, but by no means are they immune. Good cultural practices are still crucial. Resistance to these pests is usually listed on the plant label using the following abbreviations: V = Verticillium Wilt; F = Fusarium Wilt; FF = Fusarium Wilt race 1 and 2; N = Nematode; T = Tobacco Mosaic Virus; A = Alternaria (Early Blight); and TSW = Tomato Spotted Wilt.
• Septoria leaf spot (fungus): small, roughly circular spots with dark-brown borders and gray centers on leaves. Leaves may die and drop off if heavily infected. Cultural controls include planting healthy transplants, sanitation*, crop rotation, using metal stakes (or treat wooden stakes) and watering at the soil level. Chemical controls are the same as for controlling anthracnose.

*Sanitation includes removing plant debris from the garden, whether it originates in the current growing season or the previous year. Remove affected plants from the garden and destroy them so that they do not act as a source of disease-causing microorganisms. Discard any plant, transplant, or seed piece that does not look healthy. Diseased plants should not be added to home compost piles; the temperature reached in most home compost piles is not high enough to kill plant pathogens.

There are a number of additional diseases and physiological disorders. For more information, including color pictures, see 'Tomato diseases and disorders' from the Department of Plant Pathology by Iowa State University. http://www.extension.iastate.edu/Publications/PM1266.pdf

Common Diseases of the Home Garden by MU Extension provides descriptions and recommended control practices for the popular vegetable crops in Missouri and their most common diseases. http://extension.missouri.edu/explore/agguides/hort/g06203.htm