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.

The vascular system in a plant serves to move water and nutrients up through the plant and to redistribute food from the leaves throughout the entire plant. Pathogens that infect this region disrupt the movement of water and food, resulting in reduced growth, wilting and even plant death. Similar symptoms can also be caused by certain diseases that affect only the roots, like root rots caused by Rhizoctonia or Pythium species, or those caused by nematodes (microscopic worms that colonize plant roots). An accurate diagnosis begins by cutting the stem longitudinally down through the crown and roots to observe the infected areas of plant tissue. Symptoms of wilting alone are not sufficient for making a diagnosis without additional inspection of the plant. In most cases where resistant varieties are not available, or soil fumigation is not practical or desirable, avoiding areas with a history of vascular wilt or root diseases will be necessary. Small-scale production and home gardeners could also consider producing plants in raised beds or large containers filled with a ‘clean’ soil mix.

Fusarium wilt, caused by the soilborne fungus, Fusarium oxysporum f.sp. lycopersici (FOL), has a long history in Florida. It can be an extremely difficult pathogen to control because the fungus produces specialized, highly resistant survival structures that can persist in the soil for many years. These survival structures are stimulated to germinate by the presence of roots from nearby tomato plants. The fungus colonizes the root surface, eventually entering and disrupting the vascular tissues of the root leading to wilt and eventual death. Symptoms can begin within days after planting, leading to stunting and seedling death. However, symptoms of Fusarium wilt don’t typically develop until the plant reaches full maturity often corresponding with a heavy fruit load. Symptoms initiate on the lower leaves with extreme chlorosis of affected foliage, almost a golden yellow, prior to wilting. Symptoms are typically not uniform but “one-sided”, often developing in a single stem or one side of the plant; even one side of a leaf can be chlorotic and wilting while the other half remains relatively normal. These symptoms quickly progress from lower to upper foliage, and cannot be overcome with additional watering. A vertical cut of the affected stems will reveal a brown discoloration of the xylem tissue (water-conducting tissues) near the outside of the stem (just below the outer stem surface) of affected suspect plants and examine the water-conducting tissue.
The use of resistant tomato varieties is the easiest way to control Fusarium wilt. Three distinct races of FOL exist, for which most conventional tomato varieties carry resistance to FOL races 1 and 2. However, FOL race 3 is widely distributed in Florida and relatively few conventional varieties carry resistance to this race. Unfortunately, most heirloom tomato varieties (also referred to as open-pollinated varieties) are not resistant to any of the three FOL races. Purchase VFN tomato varieties, which are resistant to Verticillium (a wilt fungus similar to Fusarium), Fusarium (usually races 1 and 2), and root knot nematodes. If the use of a resistant variety is not an option, maintaining a higher soil pH and avoiding ammonium-based fertilizers have been found to reduce the severity of Fusarium wilt. Avoiding areas with a history of Fusarium wilt is the best tactic.

Bacterial wilt, caused by the soilborne bacterium Ralstonia solanacearum, is another serious vascular wilt disease of several hundred plant species, including many economically important solanaceous crops, such as tomato, pepper, potato, and tobacco. The pathogen is quite diverse and divided into several races and biovars based on host range and biochemical differences.
Diseased plants usually occur in areas associated with low lying areas where water accumulates. Initial symptoms consist of wilting of upper leaves on hot days followed by recovery throughout the cooler evening and early morning hours. Infected plants often develop adventitious roots along the lower stem above the soil line. Wilted leaves maintain a pale green color, and remain attached to the plant as disease progresses leading to irreversible wilting of the entire plant.
The vascular tissues in the lower stem of wilted plants show a dark brown discoloration of the vascular and pith tissues. The cross-sectioned stem from a plant with bacterial wilt will produce a white, milky strand (ooze) of bacterial cells when placed in clear water. The pathogen can survive on the roots of many non-host plants, including weeds. The disease rarely occurs in calcareous soils with a high pH. Moderate pH and moderate-to-high temperatures are associated with longer bacterial survival in soil.
Infested soil is the main source of inoculum. In recently cleared land in tropical and subtropical regions, it is not rare to find bacterial wilt in the first crop. Disease-free areas can be infested through infected tomato transplants, contaminated irrigation or surface water, machinery, and other cultural practices.
Bacterial wilt is very difficult to control after it is established in the field. Cultural practices might reduce the disease incidence. Seedlings must be pathogen free. It is essential that gardeners use irrigation water that is not contaminated with the pathogen. Avoid excessive irrigation as the excess soil moisture favors disease development and spread. Crop rotation with nonsusceptible crops reduces soilborne populations of the bacterium. Shifting planting dates to cooler, drier periods of the year can also be an effective strategy to escape disease development. The use of bacterial wilt-tolerant tomato cultivars (Neptune and FL 7514) could reduce the impact of this disease.

Tomato Spotted Wilt Virus (TSWV) is transmitted from diseased to healthy plants by the Western flower thrip, an insect that commonly feeds inside many different types of flowers, including the blossoms of tomato.
TSWV has a wide host range, producing symptoms in at least 63 commercially grown plants in Florida. These include vegetables, field crops, and ornamentals. TSWV is of more concern in North Florida, presumably because the thrips species and weed hosts that are best adapted to the spread and survival of this virus are more abundant in north Florida. However, TSWV may occur in South Florida too.
Symptoms of TSWV can vary depending on the tomato variety and plant age at infection. Small, light brown flecks will first appear on newly developed upper leaves. These spots later turn brown to dark purple, followed by a general bronzing of the leaves that eventually die and appear drooped on the stem. Brown to purple brown streaks often develop on the stem as well. Plants are often stunted and, with the droopy leaves, have a wilted appearance. Green fruit show large concentric yellow or brown rings, while the rings are a striking brown to black on ripening fruit.
Vigilant weed control may reduce the incidence of TSWV on your garden tomatoes but will not eliminate TSWV. Control of thrips with insecticides may help to reduce late infections (secondary cycle). However, in controlled experiments, insecticides have not been all that successful. Ultraviolet (UV) reflective mulch, used as a physical repellent, can reduce TSWV incidence. UV-reflective mulches are available commercially in small packages for homeowners.

Tomato Yellow Leaf Curl Virus (TYLCV) was first introduced into Florida in 1997 and is a constant problem in commercial fields throughout Florida. This virus is transmitted by a species of whitefly. Severe symptoms occur on tomato, especially when young plants are infected. Young, diseased plants become severely stunted causing the tops of plants to appear bushy. Leaf edges curl upward and appear mottled (alternating areas of light and dark green) with a distinct yellowing of the leaf margin. As the symptoms progress the affected leaves develop a pronounced yellow mottling Fruit set is often poor or nonexistent.

Control of TYLCV is difficult. Varieties with resistance to TYLCV are available at local garden centers and should be used, especially if you reside in areas with significant commercial tomato production. Successful control begins with the purchase of TYLCV-free transplants. Suspicious transplants that appear to be symptomatic should be removed and destroyed in an effort to eliminate sources of virus that might infect other tomatoes. A lengthy period of time between plantings in the garden will help break the cycle that can lead to repeated virus infection. Control of whiteflies is essential to prevent the virus from quickly spreading from diseased to healthy tomato plants.