Pest and Disease Control Strategies
There are a number of potential diseases, insect and nematode pests that can cause significant damage to tomatoes (see list at end of article). Pest damage can range from decreases in fruit yield or quality to plant death. This article outlines the three general strategies for managing these pests. In the reference section, Cornell has an excellent key to help identify disease pests based on plant symptoms 1.
There are multiple ways to manage tomato plants to reduce the likelihood of disease infection. As in all vegetable crops, a basic method of pest avoidance is crop rotation. Virtually all soil-borne pathogens and nematodes overwinter and will build up from year to year if a suitable host is present. Crop rotation breaks that cycle. A general rule for tomatoes is to use a 3-year rotation. This does not apply when plants are grown in containers in greenhouses/high tunnel production and new media is used every year.
Another general method is to manage tomato plants so that the environment is less favorable to disease development. Since most pathogens require humid/moist conditions for infection and/or disease development, a combination of plant spacing and pruning can be used to encourage good airflow in the planted area. This is true for both open field and protected cultures.
The pathogens that cause many leaf diseases (e.g. Septoria leafspot) overwinter on plant tissue on or just below the soil surface. The disease spreads by water droplets that carry spores, either from the soil to the plant or from older diseased leaves to younger leaves/stems on the same plant or from an adjacent plant. Disease movement from the soil to the plant can be greatly reduced by mulching. Removing leaves at the base of the plant can decrease disease movement from older to young plant parts.
As a rule of thumb, tomato plants should be watered from the base of the plant, not as an overhead sprinkling, as to avoid pathogen spore distribution through water drops. One of the reasons that there is generally less disease pressure in protected culture is that the plants are shielded from raindrops, and watering is often via drip irrigation systems.
A good understanding of what pests are prevalent in your growing area, and the planting varieties resistant to those pests, is the first line of defense for the control of tomato diseases and nematode pests.
The term “resistance” implies a single gene in the plant that provides good protection for a single pathogen. Note that with some diseases (e.g. Fusarium wilt) there are multiple races of the pathogen, and multiple disease resistance genes may be necessary to control all races. In most cases, resistance genes provide adequate control of the pest without requiring any other management input. Pest resistance genes have been primarily introduced into cultivated tomatoes by crossing to wild relatives that carry the desired pest resistance trait. Resistance genes and molecular markers for those genes are now available for over 20 key tomato pests.
There may be differences in disease reaction among varieties that are not resistant to a particular pest. Varieties that have a less severe reaction to a pest may be called tolerant. However, tolerance may be overcome when disease pressure gets severe. Heirloom types generally lack resistance genes for key diseases, but some may show some disease tolerance based on the multiple years of selection/development in a particular area. For tolerance to be effective, it will likely need to be combined with the cultural “best practices” mentioned above.
Grafting is a process in which a disease-resistant rootstock is grafted to a second variety that lacks disease resistance but produces the desired fruit. This top part of the grafted plant is called the scion. There are multiple references and YouTube videos that describe the grafting process for tomatoes. It’s possible for growers to make their own grafted plants, and several seed companies sell seed of disease-resistant rootstocks. Several seed/plant companies also sell finished grafted plants. Note that grafting can only be effective in controlling root diseases (e.g. Fusarium wilt), not those diseases that solely affect the upper plant (e.g. Late Blight). There is a more detailed discussion on grafted plants in the Tomato Variety section.
Most state university extension programs list the diseases and nematodes most prevalent in the state. The Cornell Plant Pathology Department keeps a website that lists which varieties are resistant to which pests 2. Virtually any seed company selling a variety with resistance to one or key pests will list the pest resistance ratings as part of the description. A list of the abbreviations used to note resistance to particular pests can be found here 3. For example a variety with a pest resistance description of VFF or Va/Vd, For(1-2) means it has resistance to Verticillium wilt, and Fusarium wilt (races 1-2). It is safe to assume that if there is no such resistance claim made in the variety description, the variety is not resistant. There are no formal methods to measure or report tolerance to particular pests, though such claims may also be in variety descriptions.
Although for most growers chemical control is the least favorable option for disease control – it is an important tool for commercial growers in areas where diseases are prevalent, and for which there are no good options in resistant varieties. Septoria leaf spot is a good example of a very serious disease with very limited options for genetic resistance.
Chemical control of pests in all crops should be used according to Integrated Pest Management/IPM principles. This includes rotation of fungicide active ingredients to prevent the development of new races of the pathogen with resistance to a particular fungicide. Some states recommend specific IPM programs for tomatoes 4.
Chemical control options for organic producers are fairly limited, but there are OMRI-approved options that can be effective.
KEY TOMATO PESTS:
Tomato diseases can be divided into three categories: root diseases, leaf/stem diseases, and viruses.
The root diseases are primarily fungal pathogens that plug the plants vascular network (vascular wilts) and include Verticillium wilt and Fusarium wilt. In both cases, infected plants will often have vascular discoloration evident when the root is sliced. There is good resistance to Verticillium and all three races of Fusarium wilt in some varieties. Fusarium root and crown rot is more commonly a greenhouse problem and can be most effectively controlled with resistant varieties.
There are a number of pathogens that infect leaf/stem tissue. In open-field production Septoria leaf spot, early blight and late blight are probably the most damaging. All three diseases, singly and in concert, are capable of almost complete plant defoliation. Some combination of resistant and/or tolerant varieties, the cultural practices mentioned above, and chemical control may be required to control leaf disease losses. Bacterial spot is an important pest in the southeastern U.S. with both leaf and fruit symptoms. The University of Florida outlines an integrated pest management strategy for control of Bacterial spot that includes cultural, biological, and chemical control components (reference). In protected culture, the most common leaf diseases are leaf mold and powdery mildew – both of which are less commonly seen in open field cultivation. There are varieties resistant to both of these pests.
The most prevalent tomato virus diseases are Tomato Mosaic Virus (ToMV), Tomato Spotted Wilt Virus (TSWV), and Tomato Yellow Leaf Curl Virus (TYLCV). There is good resistance available for all three of these viruses, though Sw5 resistance to TSWV is breaking down in some parts of the world. TSWV (thrips) and TYLCV (whitefly) require an insect vector to spread the disease and infect plants. Controlling the insect vector is a challenge in both open field and controlled culture production systems. These viral diseases are limited to areas where the insect vector can overwinter. ToMV belongs to the Tobamovirus complex and is most commonly spread from plant to plant by human contact, either directly with handling or with using contaminated sheers in pruning, etc. There is no required insect vector. Most modern varieties with resistance to ToMV are also resistant to the related virus, Tobacco Mosaic Virus (TMV). Tomato Brown Rugose Fruit Virus (TBRFV) is also Tobamovirus, but resistance to ToMV does not provide protection. TBRFV has just recently been identified as an important tomato pest but is still mostly isolated to parts of southern Europe, Mexico, and most recently California.
Plant-parasitic nematodes are small microscopic roundworms that live in the soil and attack the roots of plants. The root-knot nematode is the most important nematode pest in tomatoes. Nematode-induced production problems generally arise from a reduction in root mass and the related decreased efficiency of water and nutrient uptake. Root-knot nematodes have a very wide host range, so crop rotation options are very limited. Many newer tomato hybrids have good resistance to this pest.
A University of Maryland publication lists the common insect pests of tomatoes (open field production) and suggests control options (reference). It’s a long list, but chemical control of insect pests in open-field tomato production is relatively rare. Insect pests can be more often a problem in protected culture. Increasingly biological control methods are used to control GH pests 5. There is currently no host plant resistance to any key tomato insect pest.
There are several ways to combat these issues with and without the use of chemical controls.