Always in demand, tomatoes can be profitable crops for Florida’s farmers. However, tomatoes are susceptible to a number of diseases that can erode profits if they get a foothold in the crop and are not suppressed early in the cropping cycle. Some plant diseases are commonly introduced on transplants and can be avoided, so it is important to examine your plants carefully. If you save seed from previous tomato crops or produce your own transplants, consider surface sanitizing your seeds by treating with dilute bleach or hot water. Several protocols can be found on the web. However, be sure to test the protocol on a small batch of seed first, as tomato varieties can differ in their sensitivity to bleach or hot water.
Establish the crop in an area that has been rotated out of tomato production for a year or longer. Be aware that residue from previous tomato production, any volunteer tomato plants, and some weeds can harbor tomato pathogens. Avoid low-lying areas of a field or fields that had previous problems with diseases caused by soilborne pathogens or pests, unless soils were treated for such problems.
The Basics Of Plant Disease
Plant Pathology is the study of plant diseases, which are caused by microscopic organisms called pathogens. Plant pathogens are different from other microorganisms because they infect and cause disease on living plants. A plant is considered susceptible if it can be infected by a specific pathogen resulting in disease; such a plant is often referred to as a host plant. Host range refers to the types of plants susceptible to a specific pathogen. Pathogens that can infect diverse types of plants are said to have a broad host range while those pathogens that infect only a few types of plants have a narrow host range. Each pathogen requires certain environmental conditions (temperature, moisture, etc.) to successfully infect and cause disease on a host plant and, in many cases, to survive in the absence of a host plant. In order for disease to develop there must be 1) a susceptible plant, 2) a capable pathogen and 3) adequate environmental conditions for infection and disease to follow. These three requirements are typically conceptualized as the sides of a triangle and are referred to as the Disease Triangle. The basis of disease management is to disrupt any portion of the Disease Triangle to prevent or limit disease development.
Tomato is susceptible to a wide variety of diseases caused by many fungal (this includes several fungal-like organisms), bacterial and viral pathogens. A particular pathogen will cause a progression of certain symptoms on its plant host following successful infection. Leaf spotting, cankers, stunting, wilting and death are all examples of symptoms caused by pathogens. Understanding the life cycle of each pathogen (how it reproduces and how it moves in the environment) and the environmental factors that favor infection and disease development are useful tools to use when choosing disease management strategies. The goal is to use an integrated approach to eliminate as many parts of the Disease Triangle as possible. For example, some pathogens reproduce by disseminating spores that spread around the crop through rain splashing. Two management options for consideration would be to 1) use drip irrigation instead of overhead irrigation to minimize disease spread and 2) increase disease scouting and/or alter fungicidal spray programs when the incidence of rain events is high.
Tomato, pepper and eggplant are all members of the Solanaceae plant family. As a result, many of the same diseases are pathogens of two or three of them. It is not uncommon for a pathogen’s host range to include plants that are within the same plant family. Destroying a crop immediately after harvest by turning it into the soil, along with removing solanaceous weeds and rotating to a non-host crop, can reduce the carryover of a pathogen from one season to the next.
Some examples of tomato diseases and their life cycles are listed below. Additional resources that provide more complete information are referenced at the end of this article.
Bacterial spot is caused by four species of Xanthomonas; X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri, which at one time were collectively referred to as X. campestris pv. vesicatoria. In Florida, Xanthomonas perforans is the primary cause of bacterial spot on tomato, and occasionally by X. euvesicatoria. To date, neither X. vesicatoria nor X. gardneri have been found in the state. Bacterial spot is common in warm (> 75 °F), humid weather. Hence, Florida’s hot weather and heavy seasonal rains can lead to severe bacterial spot outbreaks. All plant parts are affected, but the foliar symptoms on the leaves are most noticeable.
Leaf symptoms consist of dark brown leaf spots (lesions) (~ 1/8 of an inch diameter) that extend from the lower to the upper leaf surface and have a wet-to-greasy appearance. There may be some yellowing or chlorosis surrounding these lesions. Lesions caused by X. perforans will often develop a shot hole appearance as the necrotic tissue in the center of the lesion drops out. Foliar lesions often coalesce leading to a general chlorosis of the affected leaf area and eventual blighting of the leaf. Blighting is especially severe around the margins of leaves. It is not unusual for affected leaves to drop off plants prematurely. High disease pressure can cause severe defoliation that exposes fruit to the elements, leading to sun scalding of fruit and disease from secondary pathogens.
In Florida, fruit lesions are rare but quite distinct. Lesions often start as small whitish raised blisters on green fruit. As these lesions mature, they become brown-to-black scab like spots, often with a light-colored halo of water-soaked tissue (green fruit) or green tissue (red fruit). With time, the halos can disappear as fruit lesions enlarge. The lesions can occasionally progress in size becoming sunken in the center, but will always have a raised rim on the outer edge of the lesion.
Bacterial spot of tomato, like most bacterial diseases, is difficult to control once introduced into the field. Therefore, do all you can to prevent the introduction of the pathogen. Practice good crop rotation; don’t plant your tomatoes in the same location as the previous season. Also ensure that tomato debris from the previous season is destroyed, as it is a good source of many pathogens. Purchase seed from a reputable seed company. If producing your own transplants or saving seed from an open-pollinated tomato variety, consider surface sterilizing the seed. Plants purchased from retail nursery outlets should be inspected carefully for symptoms of bacterial spot to avoid the purchase of diseased transplants.
Avoid overhead sprinkler irrigation as much as possible. Consider planting later in the fall in South Florida to minimize production in the warmer, rainy season. Increase plant spacing to help foliage dry quicker and avoid handling plants while the foliage is wet to minimize movement of the pathogen. Foliar sprays consisting of a copper-based fungicide mixed with mancozeb (can be purchased at garden centers) may provide some control of bacterial spot, but must be initiated preventatively before disease becomes well established. Unfortunately, copper resistance is quite common among many bacterial pathogens of plants, including bacterial spot of tomato. Copper applications may be inadequate to control bacterial spot, especially when weather conditions are favorable for rapid disease development.
Target spot is caused by the fungus Corynespora cassiicola, which has a broad host range that includes a diverse collection of hosts that notably includes leguminous, cucurbitaceous and many common ornamental plant species. Some isolates of C. cassiicola show some host specificity, while others are quite broad. In addition, isolates vary in aggressiveness on many hosts, including tomato. Like most foliar fungal pathogens, C. cassiicola produces abundant spores that are readily spread by wind from remote locations. C. cassiicola spores typically require free moisture (rain or dew) for 1 to 4 hours for optimum infection.
Target spot can affect foliage, stems and fruit of tomato. Foliar and stem symptoms begin as small, brown to black spots with light brown centers and dark margins. There may be yellow halos around these spots. These initial lesions are extremely difficult to differentiate from bacterial spot lesions and require additional laboratory testing to properly diagnose. However, as target spot progresses, the lesions enlarge and develop diffuse concentric rings within the lesion surrounding a tan to dark brown center. These lesions can often coalesce, leading to blighted areas on a leaf or along the leaf margin, eventually blighting the entire leaf. Often the pathogen will establish on lower, older leaves before progressing into the interior of the plant canopy where the dense foliage increases leaf wetness favoring spore germination.
Fruit lesions caused by the target spot fungus are fairly distinct and easy to distinguish from lesions caused by bacterial spot. Fruit symptoms begin as small, slightly sunken brown to black flecks usually at the hip or sides of the fruit; rarely near the fruit scar or under the calyx. As lesions expand they become darker and deeper with concentric rings within the lesion. These lesions can overlap, resulting in large, pitted areas. As fruit ripen, large sunken areas are evident, often with a gray or black growth of the fungus in the lesion center. These large lesions will typically crack giving a star-like appearance, which often leads to secondary infections.
Target spot is easily controlled with timely fungicide applications. If target spot is a recurrent problem in your production area, preventative applications of broad-spectrum fungicide may be necessary to protect plants initially. However, as the plant canopy increases, specific-systemic fungicides are necessary to protect the denser inner canopy. Selective thinning of the canopy may also help reduce disease pressure by reducing leaf wetness and to improve coverage with broad-spectrum fungicides. Consult with the UF/IFAS Cooperative Extension Service for recommended fungicides. Gardens planted close to commercial tomato production fields are more likely to be affected by target spot.
Early blight, caused by Alternaria solani and A. tomatophila, is a fungal disease that damages the leaves and fruit of tomato. Similar to C. cassiicola, the fungal spores are easily spread by the wind. However, unlike C. cassiicola, A. solani and A. tomatophila are limited to members of the Solanaceae.
On leaves, symptoms begin as small, pencil-point-size, dark-brown to black spots that are difficult to differentiate from bacterial spot without additional laboratory testing. However, the lesions become more distinct as they enlarge, reaching up to a half inch in diameter and larger with readily visible, concentric rings that look somewhat like a bull’s-eye. These distinctive leaf spots make early blight one of the easier tomato diseases to identify. When compared to target spot, early blight lesions are typically dark brown to nearly black with very distinct rings and lack the tan or light brown center.
Similar distinct concentric rings are seen in lesions that develop on stems and fruit. When the fungus attacks young stems, complete girdling of the stems may occur with subsequent plant death. Fruit lesions are usually at the junction of the fruit and fruit stem, often beginning under the calyx and are conspicuously sunken.
To control early blight, start with disease-free transplants and fertilize plants adequately. Inadequate nitrogen levels, in particular, make tomatoes more susceptible to early blight as it leads to premature senescence of lower leaves that are particularly vulnerable to infection. Similar to target spot, persistent problems with early blight will likely require preventative applications of broad-spectrum and systemic fungicides. Gardens planted close to commercial tomato production fields are more likely to be affected by early blight.