Plant Diseases and Controls

As I mentioned in the post Caring for your Tomato Plants, disease can completely destroy your plants if it's not recognized and controled. Because understanding how diseases get started and how you can control it is such an important factor in the success or failure of raising tomato plants, I'm going to give more detail.

There are 3 factors that determine if your plants have no disease, become slightly diseased or become completely overtaken by disease. In plant Pathology, those factors are known as the disease triangle and they are 1. The Host Plant 2. The Pathogen Agent 3. The Environmental Conditions.

There must be an interaction between all 3 of these factors for disease to develop. The interaction between these 3 factors may not overlap at all which means no disease, they can slightly overlap for slight infestation or all three factors can overlap each other completely which will mean major disease problems. If no disease develops it could be because the plant is resistant to the pathogen, the agent was not present or environmental conditions were unfavorable.

First, concerning plant resistance, no plants with resistance to Tomato Blight exist that I know of however plants and seed are available with resistance to other common diseases. When you purchase your plants or seed, you should always look for the disease resistance designation on the plant ID tag or the seed pack. This designation will be VFFN, VFFNT or something similar. V=Verticillium Wilt, N=Nematodes, F=Fusarium Wilt strains, T=Tobacco Mosaic. Some of these diseases are more common in certain areas of the country than others.  I do know that Verticillium Wilt is common in Iowa but most Hybrids are resistent.  However, keep in mind that many Heirloom varieties of tomatoes are not resistent to disease. 

Second, concerning the presense of the pathogen agent, if the plant is affected only slightly by disease, it could mean that it was somewhat resistant to the agent, the agents were low in number or the weather conditions were present for only a limited time. Obviously if the disease is severe, the plant was not resistant, the agent was in high numbers and favorable environmental conditions existed for an extended time period.  Keep in mind here that Verticillium Wilt, Fusarium Wilt and Early and Late Blight are all soil borne diseases.  In other words, once you get them in your soil, you will have those disease pathogens in your soil every year.

Third, concerning environmental conditions, the important factors are temperature and moisture. Temperature can stimulate or reduce the growth of a pathogen which will result in more or less disease. Fungi spores need moisture to germinate so if you have hot, dry conditions, it eliminates one of the factors and the disease can’t get started. I live in eastern Iowa and typically we have cool wet springs with humid summers so usually sufficient moisture for some degree of disease exists. Consequently, I battle tomato blight most of the growing season which means regular spraying with a good fungicide.  (see Caring for your Tomato Plants)

Parasitic funguses such as the ones that cause early tomato blight or powdery mildew depend on the host plant for their existence. Once the fungus comes in contact with the host plant, it will rob the host’s carbohydrates and other materials while, at the same time, kill the host plant cells. When the fungus reaches the reproductive stage, they will release spores and spread on the original host plant plus spread to new, unaffected host plants. If the problem is not immediately addressed, it will spread to all of your tomato plants.

Bacterial pathogens are different than fungus pathogens in that they cannot penetrate into a plant. They can only infect a plant through an opening or wound or through insects during their feeding. The symptoms of bacterial infections on plants include wilting, rots and galls.

Nematodes are small worms that normally feed on plant roots and cause swelling, knots or rotting of the roots. The plant symptoms include yellowing of the leaves or stunting.  I have never had a problem with Nematodes on my tomato plants.  The probable reason for that is I always plant Hyrbids that are resistent to them.

Viruses, like bacterial pathogens, can only enter the host plant tissue through a wound or insects. The most common symptom of a virus is stunting with obvious signs being mosaics, ring spots, leaf curl, yellowing or other leaf distortions.

Now that you have a basic understanding of what pathogens are and how they can affect a plant, we can consider some prevention methods.  Remember, all 3 factors of the disease triangle need to interact in order for a disease to be viable.  Eliminate 1 of the factors and the disease can’t get started.

With that in mind, here is a general list that can help in preventing diseases in your garden.

1. Remove infected plant material from your garden. Many diseases like fungi and nematodes can over winter in your soil.  If you have any plants in your garden that have been infected with a disease, tomato plants with blight for example, it’s important to remove the infected vegetation, such as spotted leaves, immediately and then remove the entire plant at the end of the growing season.  Take the material far away from the garden and don’t place it in your compost pile.  Of course, next season, you should start treating the plant with a good fungicide before the disease gets started.
2. Environmental Controls. We can’t control the weather but we can control the methods that we use to care for the plants.  If you have a dry growing season, Mother Nature will take care of your disease problem for you as long as you don’t provide favorable conditions for the pathogen yourself.  During dry periods when you manually water your plants, make sure you water them at the base of the plant so you don’t get any water on the foliage and water in the morning so any excess water will dry during the day.  Make sure you space your plants so they don’t hold moisture.  When I plant my tomato plants, I space them 5’ apart so after a rain or during humid weather with high dew points, they will have good air circulation and exposure to the sun so the foliage will dry quickly.
3. Insects.  As mentioned earlier, some diseases are introduced to the host plant through insect penetration.  If you have insect problems, treat it with an organic pesticide like Bulls-EyeTM Bioinsecticide but follow instructions carefully.  I occasionally have a few tomato horn worms that will chew on a few tomatoes but the damage is minimal so I don't bother to do anything.
4. Plant Protection.  As I mentioned, you can protect your host plant by purchasing hybrid plants that are certified to have disease resistance.  An example of this is Big Beef Hybrid (VFFNT) Tomato Plants.  In this example, it means that it is resistant to Verticillium (fungus) wilt, 2 forms of Fusarium fungi, root knot Nematodes and Tobacco Mosaic Virus.  Non hybrid Heirloom varieties do not have resistance and, without treatment, would be very susceptible to diseases if the pathogens existed in their environment.
5. Plant care and handling.  As I pointed out earlier, bacterial and virus pathogens need an opening, such as a wound, to enter the host plant tissue.  Whenever you’re working around your plants, such a weeding, spraying or simply placing the branches of your tomato plants through the cages be very careful not to damage them.

For more information, visit my website http://www.rmarketplaceonline.com/raisingtomatoes.html

Sharing my experiences to help you with yours!
Tomato Rog