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As home and market gardeners begin harvesting tomato fruit they will also begin to notice some of the many disorders that can affect the appearance and quality of the fruit. In this article, I will attempt to describe some of the more common diseases and disorders encountered when growing tomatoes in Wisconsin.
Blossom end rot is found throughout Wisconsin. Early symptoms include water-soaked spots at, or near the blossom end of mature fruits. The spots enlarge rapidly and may coalesce to form extensively damaged areas. As the damaged tissue dries and shrinks, the lesion surface becomes shrunken, leathery, and dark brown to black. Affected areas are commonly infected by a secondary organisms that will cause soft rot and fruit decay. Internal discoloration and tissue collapse may be present without the characteristic symptoms being visible.
Blossom end rot is a disorder associated with the temporary shortage of available calcium in the fruit. It is aggravated by soil moisture fluctuations. If water is limiting, plant growth slows and subsequently, nutrient uptake is reduced. This reduces calcium concentrations in the plant since calcium is carried through the plant in the water flow. Because calcium is carried along in the water stream, those plant parts that are transpiring most rapidly will receive the most calcium. This is often the oldest leaves since they are the largest. Developing fruit is least likely to receive the calcium it needs. Moisture stress that results in even partial wilting of the crop is enough to cause a calcium deficiency. Once moisture is available, the plant resumes regular growth but calcium uptake continues to lag and the rapidly-expanding plant doesn’t have enough calcium to develop properly. Applying nitrogen as ammonium also appears to directly affect calcium uptake by tomato plants. This produces a reduction in calcium absorption and accumulation in the tomato leaf tissue.
The incidence of blossom end rot is reduced by providing even moisture and avoiding excessive nitrogen fertilizer applications. Although varieties do differ in susceptibility, there are currently none with sufficient tolerance to provide dependable control. Timely and repeated spraying of tomato fruit and foliage with a dilute solution of calcium chloride has been reported to prevent blossom end rot but the benefits are often variable.
During hot, sunny weather, growers may also notice sunscald on green or breaker tomatoes. This disorder appears as a white or light tan discoloration of the fruit that has been exposed to sun. Discolored areas are sunken and wrinkled and often surrounded by a yellow halo. Damage is typically found on the top side of the fruit as this is the area most likely exposed to the sun. Sunscald frequently occurs in tomato when the lower leaves drop off due to Septoria and early blight exposing the fruit to intense sun. Secondary fruit rot organisms may invade damaged tissue.
Catfacing describes the presence of deep indentations in the blossom end of the fruit. In some cases, the fruit itself is misshapen, becoming kidney-shaped or otherwise distorted. This damage occurs when temperatures drop below 50 degrees F during flowering and fruit set, resulting in poor pollination. In some cases, excess heat, 2,4-D injury, and erratic soil moisture can lead to catfacing. High nitrogen has also been shown to aggravate this disorder. Catfacing is cultivar specific and appears more frequently on older cultivars. Large-fruited cultivars are most susceptible.
Two types of growth cracks affect the stem end of tomatoes: concentric and radial. Concentric cracking produces circular cracks around the stem end of the fruit. Radial cracks spread outward from the stem scar. These cracks typically appear as the fruit matures. Growth cracks often appear when conditions drastically change the rate of growth, such as wide fluctuations in temperature and moisture. Dry weather followed by heavy rains causes radial cracking in many tomato cultivars. Cultivars vary in their ability to withstand cracking depending on the strength and "stretchability" of the skin. Very susceptible cultivars will crack while still green, those that are somewhat resistant often don’t crack until the fruit has reached the breaker stage. The earlier the fruit cracks, the deeper the cracks become. High nitrogen and low potassium are often implicated in fruit cracking and therefore, proper plant nutrition and adequate, regular irrigation will reduce the likelihood of growth cracks.
As the name implies, fruit suffering from puffiness appear somewhat bloated and angular. When cut, cavities may be present that lack the normal "gel" and the fruit as a whole isn’t as dense. Puffiness results from incomplete pollination, fertilization, or seed development often as a result of cool temperatures that negatively impact fertilization. Similar to growth cracking, high nitrogen and low potassium can also lead to puffiness. Some tomato cultivars are more susceptible to this disorder than others.
Blotchy ripening of tomatoes is characterized by areas of the fruit that fail to ripen properly because of poor fertilization. This problem is more prevalent in cool weather and is exacerbated by too much or too little water. White or yellow blotches appear on the surface of ripening fruit while the tissue inside remains hard. The affected area is usually on the upper portion of the fruit. This disorder may be confused with symptoms produced by the tobacco mosaic virus. Blotchy ripening is cultivar specific and appears more frequently on older cultivars.
There are also several fruit rots that affect tomatoes after the fruit has been picked. Below is a description of the common maladies growers may confront.
This fruit-rot is more likely to develop on ripe or overripe fruit late in the season. Infection occurs when the developing fruit is still small but symptom expression doesn’t appear until after the fruit is harvested. Lesions are circular, approximately ½ inch in diameter, and slightly sunken. The internal tissue beneath a lesion is often grainy and lighter in color than surrounding healthy tissue. The center of the lesion often contains small, black, specks that represent the reproductive structures of the fungus. Secondary infections are common in fruit infected with anthracnose and as a result, multiple fruit rots may be present concurrently. Infected tomatoes don’t store well.
Colletotrichum coccodes is the most common species of fungi to cause tomato anthracnose. This pathogen is seedborne or may overwinter in the field on infected debris. Overhead irrigation and wet weather worsens the disease. It is important to use clean seed, crop rotation, and plant spacing which allows optimum air circulation and promote drying of the plants after irrigation, rainfall, and heavy dews. Fungicides applied to prevent septoria leaf blight will also protect fruit from anthracnose.
There are two common diseases of tomato caused by bacteria that result in fruit problems —bacterial spot and bacterial speck. Bacterial spot produces raised, scabby lesions on the fruit when environmental conditions are warm and wet. When the symptoms first appear they may be confused with bacterial speck due to their minute size. However, as symptom expression develops, the lesions enlarge and become raised. Unlike the lesions of early blight, those produced by bacterial spot do not consist of concentric rings. The organism responsible for the disease is Xanthomonas campestris pv. vesicatoria. Crop rotation, use of clean seed or seed treatment prior to planting, and protectant bactericides will reduce the likelihood of this disease.
Bacterial speck produces tiny, slightly raised, superficial black specks that are less then 1/16 inch in diameter. There is often a darker green ring around the lesions. Lesions produced on fruit suffering from bacterial speck are much smaller than those of bacterial spot. The pathogen, Pseudomonas syringae pv. tomato, overwinters in the field on infected plant debris. The disease may also be transmitted on infested seed. This is a cool weather disease and occurs when temperatures persist between 55-77 degrees Fahrenheit with high humidity, heavy dew, or frequent rainfall. Bacterial speck, in addition to reducing yield through cosmetic injury, also reduces yield by up to 25% when leaf infection occurs. The use of clean seed and irrigation early in the day to promote drying of the plant before nightfall will reduce the incidence of infection. Crop rotation and sanitation to reduce the current season’s fruit from exposure to last season’s inoculum is also important in preventing bacterial speck. Fixed copper fungicides that are used to control foliar symptoms of this disease will also protect the fruit from infection.
Grey mold, as the name implies, appears as a grey, velvety coating on flowers and fruit. This disease can affect foliage, flowers, and fruit. Senescing flowers are very susceptible to the grey mold fungus and when these infected flower parts contact the fruit, fruit infection results. Ghost spots, which are circular, whitened areas with distinct rings develop on green fruit. If cool, overcast conditions predominate, the disease progresses and a fuzzy grey mold develops. If conditions are warm and sunny, only the ghost spots remain without any further disease progression. The fungus that causes infection, Botrytis cinerea, is ubiquitous and affects many species of plants. Cool, wet weather worsens the disease. Fungicides can be used to prevent the spread of infection.
This disease commonly affects both fruit and foliage. Lesions are black or brown, firm, ¼-½ inches in diameter, and slightly sunken. Upon close inspection of the lesion you will notice concentric rings. Fruit lesions often begin near the stem end of the fruit. Early blight lesions are often confused with those of Alternaria listed below since both are caused by fungi in the same genus–Alternaria. Early blight is caused by Alternaria solani while alternaria fruit rot is caused by A. alteranata f.sp. lycopersici. Cool, humid conditions precipitate disease development. The fungus overwinters in the soil on infected plant residue and can remain for several years until tomatoes are planted in that field again. Disease-free seed should be planted and crop rotations practiced. The use of plastic mulch will prevent spores from being splashed up onto the fruit. In commercial production, fungicides such as chlorothalonil, maneb, or mancozeb can be used to prevent both foliar and fruit infections.
The fruit infections commonly referred to as "freckle" or "black mold" are caused by Alternaria alternata f.sp. lycopersici. When the disease begins on green fruit it is called freckle and appears as small specks that are slightly sunken and surrounded by a yellow halo. As time goes on, the lesions enlarge in size to about ¾ inch. The individual lesions are often dark with concentric lighter rings. Once the fruit ripens, the lesions stop expanding. Fruit spots are not always obvious at harvest but develop 3-5 days after harvest.
Fruit that is infected while ripe is called black mold and develops large, sunken, black lesions that may cover up to
aof the fruit. Susceptibility varies depending on tomato variety and symptoms often appear on the side of the fruit that is exposed to the sun. The disease overwinters on decaying plant debris and is spread by wind and splashing rain. Control is recommended through the use of resistant cultivars.
In recent years, late blight has become an increasing problem on potatoes grown in Wisconsin. The pathogen, Phytophthora infestans, can also infect tomato and eggplant as well as other members of the potato family (Solanaceae). This disease is more severe in cool, wet years. Fruit infections usually begin on the shoulders as the spores are dropped from the foliage above. Greasy, greenish-brown lesions are characteristic of this disease. The lesions enlarge until the entire fruit is covered. The fruit may remain firm or may soften as the disease progresses deeper within the fruit. Disease-resistant varieties exist but may not always be practical given customer preferences. Fixed copper compounds used to control bacterial spot and late blight foliar infections will prevent fruit rot as well.
Buckeye Rot
Another disease caused by a species of Phytophthora is buckeye rot. This disease is most frequently caused by Phytophthora parasitica although other species of Phytophthora may be responsible. Fruit symptoms begin as grey-green or brown, water-soaked lesions. As the disease progresses, the lesions enlarge and develop concentric rings. Lesions may cover up to one half of the fruit and are often leathery in texture. Fruit that are infected before they become mature often remain firm and green while later infections result in the un-infected fruit tissue to ripen normally. Fruit that comes into contact with the soil is most likely to become infected and infection occurs at the point at which the fruit contacts the soil. Plastic mulch will prevent the fruit from touching the soil and therefore would reduce the likelihood of infection.
