SWPAP
vegetable disease photos
Below, the reader will find a library of photos associated with vegetable diseases and other non-infectious maladies. The vegetable diseases that are listed are ones that the author has found to be important over a 28-year career in Indiana. No attempt has been made to include all possible vegetable diseases. Instead, think of the diseases listed here as vegetable diseases that are likely to be encountered in the Midwest during a typical season. If a disease seems to be missing, this probably means that I have not had enough encounters with the disease to obtain quality photos. Or maybe I have never encountered the disease. For example, many of the viral diseases that occur on vegetables in the southeast US are not described here.
An introduction has been given under each crop to briefly describe the crop in Indiana. A brief description of each disease will hopefully include a few useful ideas. Crops and diseases are listed in alphabetical order. This is a work in progress. Please address comments or questions to Dan Egel egel@purdue.edu.
All photographs were taken by Dan Egel unless otherwise noted. These photographs are in the public domain: feel free to use these photos for presentations, publications or other uses. It is not necessary to request permission. I request that any attributions mention the photographer or Purdue University. Any communication regarding these photographs after February 1, 2024 should be to (812) 886-0198.
Asparagus is not grown extensively in Indiana; therefore, I have limited experience with this crop. Of the diseases below, Cercospora leaf spot and rust are diseases of ferns. Purple spot appears on spears.
Cercospora blight of asparagus
Cercospora blight-Initial symptoms on ferns may be the chlorotic and necrotic lesions on stems. Closer observations will reveal the light oval lesions with darker margins. The causal fungus may be observed in the center of lesions (in structures known as stomata) under appropriate conditions.
Purple spot-spears may appear as a pink or purple mottle. Lesions may have light necrotic centers.
Rust-Erumpent pustules appear dark. Ferns may appear necrotic and unthrifty.
In Indiana, basil is usually planted on small acreage operations as part of herb production grown for retail.
Black leg-Dark lesions can be seen on the lower stem of plants resulting in the decline of the plant. The symptoms shown here are from a hydroponic operation.
Downy mildew-The upper leaves may exhibit a chlorosis where infection has occurred. On the underside of the leaf, a gray sporulation may be observed under moist conditions.
Gray mold-infection of young seedlings may result in the necrosis of a portion of the plant or death of the entire plant.
Beets are not planted in large acerages in Indiana. Cercospora leaf spot is the most common disease I have observed.
Cercospora leaf spot of beet-Lesions are often circular with a red margin. When numerous lesions aggregate on a leaf, severe damage can result.
Indiana is one of the top 5 or so producers of cantaloupe in the US in most years. Cantaloupe production has mostly consolidated to large acreage growers that are able to properly wash, sanitize and store cantaloupe. Sometimes referred to as muskmelon.
Angular leaf spot of cantaloupe
Angular leaf spot-Lesions of angular leaf spot may be irregular in shape and water-soaked. Lesions may be present on the margin of the leaf, perhaps due to the presence of hydathodes which may provide a mode of entry. Chlorosis may be minor. Angular leaf spot prefers cool temperatures. This is mostly a disease of transplant facilities: I rarely observe this disease in the field. Angular leaf spot is not usually an economic problem; however, it may be confused with bacterial fruit blotch.
Anthracnose of cantaloupe-The lesions of anthracnose of cantaloupe on leaves appear generally round with zig-zag margins. However, the lesions do not usually appear as jagged as those of anthracnose of watermelon. I don’t observe as many cases of anthracnose of cantaloupe as I do on watermelon. This may simply be due to the larger acreage of watermelon in Indiana. However, it is important to remember that anthracnose of cantaloupe is usually caused by race 2 of Colletotrichum orbiculare, the same race that affects cucumber. Anthracnose on watermelon is caused by race 1.
Alternaria leaf blight of cantaloupe
Alternaria leaf blight-Alternaria leaf blight has mostly round lesions with a bulls-eye or concentric ring structure, as do many Alternaria diseases on other crops. Fortunately, lesions do not occur on stems or fruit. However, in severe cases sufficient defoliation can occur to cause loss fruit of quality either by lower soluble sugars and/or sunscald on fruit. Alternaria leaf blight doesn’t seem to be an important problem most years. Fungicide schedules designed to control gummy stem blight or anthracnose seem to also control Alternaria leaf blight.
Bacterial wilt of cantaloupe-The complex biology of bacterial wilt makes managing this pest difficult. Bacterial wilt is caused by a bacterium vectored by the striped or spotted cucumber beetle. Bacterial wilt is quite common every year and can be recognized by the wilt and decline of affected plants.
Fusarium fruit rot of cantaloupe
Fusarium fruit rot-This disease was more common when cantaloupe varieties with deep sutures were frequently grown. The disease often starts with a soft area at the terminus of a suture. A white mold will often be observed in this area.
Gummy stem blight of cantaloupe
Gummy stem blight of cantaloupe-Lesions on leaves tend to appear drier than gummy stem blight lesions on watermelon leaves. Occasionally, lesions occur on fruit, in which case the disease name is black rot. Gummy stem blight has also been observed as an important disease in transplant houses. Management of gummy stem blight depends, in part, on a good knowledge of fungicide insensitivity among isolates of gummy stem blight in Indiana
Magnesium deficiency of cantaloupe
Magnesium deficiency-Symptoms are usually interveinal and include necrotic areas. Plants with symptoms are often clustered in areas where the soil pH is low, usually much lower than 6.0. Areas affected by Magnesium deficiency are usually high and well drained.
Manganese toxicity of cantaloupe
Manganese toxicity-Leaves affected with Manganese toxicity, when held up the light, will exhibit minute chlorotic pinpoint lesions. Aggregated symptoms appear as necrotic areas between the veins. Plants with symptoms are often clustered in areas where the soil pH is much lower than 6.0. Plants affected by manganese toxicity are often in relatively low areas of the field.
Phytophthora blight of cantaloupe
Phytophthora blight-For the most part, cantaloupe is not as susceptible to Phytophthora blight as, for example, pumpkin and watermelon. However, under conducive conditions, Phytophthora blight can infect any plant part of cantaloupe. In some of the photos below, Phytophthora blight has caused a lesion on the crown of the plants which has caused the plant to wilt
Root knot nematode of cantaloupe
Root knot nematode-Initial symptoms due to root knot nematode of cantaloupe may be decline or stunt. Under severe conditions, the plant may actively wilt. The roots of affected plants exhibit galls as shown here.
Combined cantaloupe disease photos
Carrots are not produced in large acreages in Indiana.
Leaf blight-necrotic lesions may make carrot ferns appear brown or black. Lesions are irregular in shape.
Root-knot nematode-infection on carrot may result in the decline of the plant. But also, since the root is the marketable portion of the plant, the galls will affect marketability directly.
Cole crops, or brassica crops, grown in Indiana mostly include cabbage and broccoli. Production ranges from relatively large acreage for wholesale to smaller retail operations.
Alternaria leaf spot of cole crops
Alternaria leaf spot-Lesions of Alternaria leaf spot on cole crops are typically round with a concentric ring structure. I have not found this disease to be very important on crops in the field in most years. But it can be devastating in transplant greenhouses.
Black rot-The importance of black rot on a cole crops depends in part on when the disease becomes prevalent. If the disease doesn’t become prevalent until late in the season, yield loss will probably be light. However, if black rot affects the crop at an early stage, yield loss may be considerable. Black rot may also be important in the transplant greenhouse. Typical symptoms include V-shaped lesions on the leaf margin.
Wirestem-The above ground symptoms of wirestem may be the decline or wilt of the plant. Once unearthed, the characteristic narrowing of the lower stem and upper portion of the root system can be observed. This portion of the stem/root system is often discolored as well.
Cucumbers production in Indiana is mostly small acreage alongside other cucurbits for retail. Increasingly, cucumbers may be grown in high tunnels or greenhouses.
Anthracnose of cucumber-Symptoms are similar to anthracnose of cantaloupe.
Bacterial wilt-The complex biology of bacterial wilt makes managing this pest difficult. Bacterial wilt is caused by a bacterium vectored by the striped or spotted cucumber beetle. Bacterial wilt is quite common every year and can be recognized by the wilt and decline of affected plants.
Charcoal rot-The first symptom one is likely to observe of charcoal rot is the wilt of cucumber plants. Upon closer inspection, the affected plant has developed a light gray canker that encircles the stem. The canker may be dotted with myriad dark, microsclerotia. The causal fungus has a huge host range and prefers warm soils. This disease has not been common, but maybe become more so in soils under high tunnels.
Downy mildew-The presence of this disease in Indiana depends on the arrival of spores of the causal fungus-like organism each year. Such spores are more likely to arrive in August or later. The chlorotic lesions of downy mildew on cucumber are angular. Multiple lesions may coalesce to encompass large areas of the leaf. Under moist conditions, the causal fungus-like organism may sporulate, becoming visible on the underside of the leaf. Stems and fruit are not directly infected.
Gummy stem blight-note dark, fungal structures (pycnidia) in center of mature lesions.
Powdery mildew-Symptoms of this disease on cucumbers are relatively easy to recognize from the white talc-like appearance on leaves. This disease appears to be more likely on cucumbers grown in greenhouses or high tunnels.
White mold-Affected plants may begin to wilt due to lesions on stems. Dark, irregularly shaped sclerotia may be found inside or outside stems and are often associated with a white mold. Fruit may also be affected. Huge host range. More common in greenhouses than in fields.
Gourds are often produced alongside pumpkins; therefore, disease management of gourds and pumpkins are often similar.
Anthracnose of gourds-Lesions tend to be dark and jagged. The lesions can be widespread on leaves, but the disease doesn’t seem to pose a problem in most years. The causal fungus may spread to fruit, but normally doesn’t cause marketable yield loss. In fact, light infections may develop in artistic patterns.
At one time, Indiana was ranked relatively high as a state with acreages of processing green beans. However, most of my interactions have been with retail/wholesale growers with small to moderate sized acres. The most significant diseases of green bean I have observed might be said to be part of the green bean root rot complex. Gray mold and white mold are sometimes observed on green beans, but are not among the diseases listed here.
Pythium root rot of green beans
Pythium root rot of green beans-Most types of root rots will be indistinguishable from above the ground: plants display symptoms of wilt and decline. Root rots that are caused by Pythium species often have a moist appearing root rot.
Rhizoctonia root rot of green beans
Rhizoctonia root rot of green bean-Above ground, plants show symptoms of decline. Root rot tends to appear not as moist as Pythium root rot.
Lettuce is usually grown in relatively small acreages for retail in diverse plantings.
Lettuce drop-affected plants may appear to have collapsed. Upon closer inspection, the dark, irregular fungal structures known as sclerotia can be found associated with the diseased plant.
Onions are a common crop grown for retail or auction, but are not grown in large acreages.
Black mold-This is not a common disease and is most likely to be discovered in or after storage.
Pepper production in Indiana maybe for wholesale or retail. Bell peppers are often grown in smaller operations for retail. Specialty peppers, such as hot peppers, are often grown for wholesale. Overall, pepper production in Indiana lags behind tomato
Anthracnose of pepper-Lesions tend to be rounded and sunken. The lesion and the area around the lesion may be soft. Under moist conditions, sporulation of the causal fungus can be observed and the lesion may have an orange or salmon color. Note that all of the photos here are of colored peppers. This may be because overripe peppers are more likely to become diseased and show symptoms.
Bacterial spot of pepper-Lesions are variable. They maybe irregular and water-soaked. Typical lesions will be necrotic with a chlorotic halo. Lesions on fruit are less common and are often raised.
Phytophthora blight of pepper-Perhaps our most serious pepper disease. Often the first indication of a problem is the wilt and death of a plant. Dead plants often can be found in low areas. Infected lower stems may be dark or even purple. Fruit may show a white sporulation of the causal fungus
Southern blight of pepper—the southern blight causal fungus affects many crops. Tomatoes may also be affected. Peppers may wilt and decline. Small white to brown sclerotia, which are fungal resting structures, may be observed at the base of affected plants.
Tomato spotted wilt virus of pepper
Tomato spotted wilt virus of pepper-I have little experience with this disease, perhaps because peppers are seldom grown in greenhouses alongside ornamentals. At least in Indiana.
Although Indiana is not known for potato production, there are several large-scale potato producers in Indiana. For the most part, these producers have their fields scouted (production is mostly for processing) and diseases are managed accordingly. I do occasionally get questions about diseases and the entries below are the result. Diseases which are well known and relatively easily managed may not be reported here. An example would be early blight which must certainly occur on potato, but I have no photos from Indiana.
Blackleg of potato-Often the first symptom one might notice is the wilt and collapse of potato plants. Upon closer examination, the stems of such plants may be dark and slimy. These symptoms may extend down into the roots and into the seed potato. Two genera may be responsible for this disease in the Midwest: Pectosporium and Dickeya. The latter genus used to be regulated.
Late blight of potato-I have little experience with this disease. In fact, the photos below are scanned from 35 mm slides.
Scab of potato-affected potatoes may appear to have a warty appearance.
Pumpkins may be produced in relatively large acreages for wholesale. Most pumpkin production is direct seeded. Increasingly, pumpkin production is no till or reduced till acreage.
Anthracnose of pumpkin and squash-The lesions on fruit can cause the fruit to be unmarketable. However, the disease is not common. I have never observed lesions on leaves.
Bacterial leaf spot of pumpkin
Bacterial leaf spot of pumpkin-Also known as Xanthomonas leaf spot. This continues to be one of the most important diseases of pumpkin. The lesions on leaves tend to be light brown and angular. However, lesions on leaves are not economically important. Leaf lesions do provide reservoirs of bacteria that may splash to fruit where they are responsible for raised, necrotic lesions often with water-soaked margins. Fruit lesions may affect marketability. In addition, lesions may be secondarily infected by fungi, creating enlarged lesions which may result in the rotting of the entire fruit.
Cercospora leaf spot of pumpkin
Cercospora leaf spot-Not a common disease nor an economically important one. However, it may be important to differentiate this disease from others such as bacterial leaf spot. Lesions on leaves are often a light brown and may be irregular in shape with no chlorotic halo. Fruit do not seem to be affected.
Downy mildew of pumpkin-Since the downy mildew causal organism doesn’t overwinter in Indiana, the disease doesn’t appear each year. However, the disease can be important if it appears in July through early September. On pumpkin, the lesions are often a mustard yellow and angular. Under moist conditions, sporulation can be observed on the underside of the leaf. When several lesions coalesce, the center of the enlarged lesion can turn necrotic. Fruit are not affected directly; therefore, late disease outbreaks may not require management.
Fusarium fruit rot-Fruit may initially seem to have a soft area. The lesion will become sunken. A white sporulation may be observed. Lesions of Fusarium fruit rot may be as a result of an initial bacterial leaf spot infection.
Gummy stem blight/black rot of pumpkin
Gummy stem blight/black rot-Gummy stem blight is the name for symptom on leaves or stems. Black rot is the name for symptoms due to the same pathogen on fruit. Gummy stem blight symptoms are not economically important on pumpkin in Indiana. Black rot problems occasionally cause marketability issues. The latter symptoms can sometimes appear as a ring structure which may appear to be virus-like symptoms. However, dark fruiting bodies in the lesions are diagnostic.
Phytophthora fruit rot of pumpkin
Phytophthora fruit rot of pumpkin-This important disease can cause fruit rot and a disease of the vine. Thus, the observer may first notice a wilt of vines. Lesions may occur on vines or on leaves. A damping-off may be observed. Fruit may become soft and develop white mold on the lesion, often toward the ground.
Plectosporium blight of pumpkin
Plectosporium blight of pumpkin-Spindle shaped, white or off-white lesions may be observed on the stem including the area known as the handle of the pumpkin. In severe cases, the fruit may be covered with scabby lesions. The occurrence of this disease is sporadic: although the disease is not common, when it appears, it can prove economically important.
Powdery mildew of pumpkin-Powdery mildew of pumpkins is easily recognized and may be observed to some extent in most pumpkin plantings. While powdery mildew is usually not severe, pumpkins that are not managed for this disease may have lower yields and/or lower quality fruit. Most growers use systemic fungicides and varieties of pumpkin with partial resistance to powdery mildew.
Virus-The most common virus diseases of pumpkins in Indiana are aphid borne potyviruses. Symptoms on foliage may include mosaic and shoestring leaves. Pumpkin fruit may be undersized, misshapen, and/or display uneven ripening patterns. The importance of Potyviruses in Indiana depends on when symptoms appear in relation to fruit set. Early infections are more likely to lead to yield or quality loss.
Spinach is usually grown for retail in small acreages. Some production occurs in high tunnels or greenhouses.
Cladosporium leaf spot of spinach
Cladosporium leaf spot-lesions are often round. Sporulation may be observed with a 10X hand lens. The outbreak in these photos was in a high tunnel.
Downy mildew-not a common disease in Indiana. Look for irregular chlorosis on the top of leaves. Sporulation may be observed on the bottom of lesions under moist conditions.
Squash is usually produced for retail markets, but occasionally for wholesale. Plants are usually produced as transplants on black plastic. Often produced as several successive crops.
Anthracnose-Concentric lines of sporulation of the causal fungus may be observed in a large sunken lesion. I have only observed this disease in post-harvest.
Bacterial spot and black rot of squash-In the photos shown here, both bacterial spot and black rot can be observed on this spaghetti squash. The bacterial spot lesions are round, 2-3 mm in diameter and scabby. Lesions of black rot are large, brown, water-soaked area in Figure 2.
Bacterial wilt-This disease is not common in squash since, as in pumpkins, only feeding of the cucumber beetle at or prior to the 5 leave stage results in disease.
Choanephora rot-May result in the rot of the blossom end of the squash. Sporulation is usually obvious.
Phytophthora blight-Symptoms similar to the same disease on pumpkin.
Plectosporium blight of squash
Plectosporium blight of squash-See pumpkins. Often occurs sporadically, but when it does appear, it can be lower fruit quality.
Powdery mildew-Not a common disease of squash—seen here in a high tunnel.
Sooty mold & fly speck of squash
Sooty mold and flyspeck-Symptoms are superficial, but may nevertheless, affect marketability.
Sweet corn is grown in Indiana both in relatively large acreages for wholesale and in small acreages for retail. Insect pests seem to be the most common complaint, but I have some experience with sweet corn disease questions.
Common rust-there are several different kinds of rusts that affect sweet corn. In most years, there is little affect on yields.
Northern corn leaf blight of sweet corn
Northern corn leaf blight-not a common or important disease on sweet corn in Indiana
Smut-In most years, more of a curiosity.
Stewart’s wilt-less important as hybrids with resistance have become more common.
I am told that there were once many more sweet potato acres in Indiana. Today, however, most sweet potato operations are relatively small.
Scurf-Symptoms include a discoloration of the skin which is mostly superficial. However, this discoloration affects marketability and cannot be determined until harvest.
Processing tomato production in Indiana is usually second in the US—a distant second to California. However, processing tomato is an important crop throughout Indiana. The most important diseases of processing tomatoes are bacterial spot and bacterial canker. White mold can also be a problem. Many of the fresh market tomatoes produced in Indiana are grown in high tunnels or greenhouses. The tomato diseases common in high tunnels are leaf mold, white mold and gray mold. Tomato spotted wilt virus, transmitted by thrips, can also be important.
Anthracnose-This disease often affects tomato fruit that are overripe and close to the ground. But this is not always the case. Sometimes caused by same organism responsible for anthracnose of pepper.
Bacterial canker-the first symptom observed is often the marginal necrosis and chlorosis on leaves. If stems are cut low to the ground, an internal discoloration is often observed. Fruit may also have a distinctive birds-eye lesion. This may be our most important tomato disease. May occur in field or greenhouse tomatoes.
Bacterial speck of tomato-This disease prefers cooler weather than bacterial spot. Often an early season disease. Typically, each lesion is associated with chlorosis, whereas bacterial spot lesions are often only associated with chlorosis when several lesions coalesce. I have only observed this in field tomatoes. Not usually as important as bacterial spot.
Bacterial spot of tomato-Important disease of field tomatoes. Water-soaked lesions are associated with chlorosis only when several lesions occur in close proximity. Lesions on fruit may be large and scab-like. Copper resistance is an important factor in managing this disease.
Blossom-end rot-This is not an infectious disease. Nevertheless, it is important to differentiate these symptoms from infectious diseases. Note leathery-like lesions on blossom end.
Buckeye rot-I have only observed this disease in processing fields. The characteristic symptom is the dark, concentric pattern of necrosis on tomato fruit.
Cercospora leaf mold of tomato
Cercospora leaf mold-This disease is known as a tropical or subtropical disease. In recent years, however, this disease has become more frequent in Indiana, particularly in high tunnels. Note dark or black sporulation in contrast with olive green sporulation common in leaf mold.
Early blight-One of the most common tomato diseases in the Midwest. Note older leaves are more severely affected. Concentric ring lesion is typical. Common in field production, but not difficult to manage. Less common in high tunnels.
Ethylene phytotoxicity of tomato
Ethylene phytotoxicity-This is not an infectious disease. May occur in greenhouses when there is inadequate exhaust or faulty heaters. Note epinasty.
Fusarium crown and root rot-Initial symptoms one might notice is wilt and decline of the plant. Lesion on base of stem is characteristic. Note vascular discoloration a few centimeters up the stem in contrast to Fusarium wilt where the discoloration continues much higher on the plant.
Fusarium wilt-One sided wilt is a diagnostic symptom of Fusarium wilt of tomato. An additional symptom is the vascular discoloration that extends many centimeters up the stem. This disease is not common perhaps due to good resistance in modern hybrids.
Gray mold-Symptoms occur on leaves, stems and fruit. Leaf lesions tend to be light brown. Stem lesions may be a darker brown. Fruit may appear soft. All plant parts may be covered with a gray mold.
Intumescence-Not an infectious disease. Symptoms occur only on plants in greenhouse or growth chamber and may consist of bumpy growth. Not an important or common problem.
Late blight-Not a common problem in Indiana since the causal fungus-like organisms doesn’t usually overwinter in Indiana. However, when this disease does occur quick action and a specific set of fungicides will be required. Necrotic leaf lesions may be ringed with the white sporulation of the causal fungus. Fruit lesions appear to have a soft brown area.
Leaf mold-Lesions of leaf mold are often chlorotic with indistinct borders. On the underside of leaves, the olive-green fungus may be sporulating. For the most part, this is a greenhouse or high tunnel disease.
Leaf roll-this symptom is not necessarily due to an infectious disease. Leaf roll may occur due to any type of plant stress. In addition, some tomato varieties are more likely to exhibit leaf roll than others. Leaf roll does not necessarily represent a problem.
Pith necrosis-this disease is not usually an important problem. Irregular necrotic lesions on the stem are characteristic. Stems may become dark and twisted. Pith necrosis is more often found in a greenhouse or high tunnel than in a field.
Powdery mildew-easily recognized by the white talc-like lesions on leaves and stems. More often observed in greenhouses or high tunnels than in the field. Not usually a problem.
Septoria leaf spot-One of the most common diseases of tomato in the field. Symptoms occur first on older leaves. Lesions are medium brown with gray centers. Dark, fungal structures (pycnidia) can often be observed in center of lesions. Lesions may have chlorotic margins.
Southern blight-Often the initial symptom of this disease is the sudden wilt and decline of the plant. Closer observation will show a lesion at soil level. Lesions will have small light brown fungal structures known as sclerotia. Causal fungus will affect many other crops. Not common in Indiana.
Target spot-Lesions often have a ring structure. Not common in Indiana. Casual fungus may affect other crops as well.
Tomato spotted wilt virus of tomato
Tomato spotted wilt virus-may be associated with diverse symptoms. Symptoms may include stunting, chlorosis, round lesions, and discolored fruit. Since the disease is transmitted by thrips, observations of these insects or feeding damage may help with diagnosis. Often the result of tomato production alongside flower plugs in a greenhouse or high tunnel.
Walnut allelopathy-Roots of black walnut trees are associated with a chemical that can interfere with the growth of many plants including tomatoes. Not an infectious disease.
White mold-Also known as timber rot when affecting tomato. Portions of the plant may begin to wilt due to lesions on stems. Dark, irregularly shaped sclerotia may be found inside or outside stems and are often associated with a white mold. Fruit may also be affected. Huge host range. More common in greenhouses than in fields.
Zippering-This is not an infectious disorder. The scar that appears along the fruit is caused when a flower drags along the embryo (fruit) surface as it enlarges.
In most years, Indiana ranks among the top handful of states in the US in watermelon production. While some farms will produce watermelon for retail or auction, much watermelon production is for wholesale for midsized and large operations. The most common foliar diseases are anthracnose and gummy stem blight. Fusarium wilt and Phytophthora blight may be the most important soil borne diseases.
Alternaria leaf blight of watermelon
Alternaria leaf blight-This disease is more common in cantaloupe than watermelon. Note dark, lesions with ring structure. Not an important disease in Indiana.
Angular leaf spot of watermelon
Angular leaf spot-Almost always observed in transplant greenhouses, this disease is seldom observed in the field. Note water-soaked lesions with irregular margins and at least some chlorosis. Disease more common in cool conditions. Not usually an important disease, however, it may be confused with bacterial fruit blotch.
Anthracnose-One of our most important foliar diseases of watermelon. Lesions on leaves are often irregular and jagged. Lesions on fruit may be sunken; under moist conditions, fruit lesions may be associated with a salmon or orange coloration.
Bacterial fruit blotch of watermelon
Bacterial fruit blotch is easily identified by the large greasy appearing lesions on the fruit, often on the top side of the fruit. It is helpful, however, to become familiar with the necrotic lesions on leaves. While these lesions are not important economically, identification of lesions on transplants or early plantings may help one to manage the disease. Leaf lesions also provide a reservoir of inoculum for lesions on the fruit. Almost always associated with seed production.
Black root rot-caused by soil borne fungal organism. Not common, but can be economically important when present. Note dark, coloration on base of stem (hypocotyl).
Cercospora leaf spot-Not a common or economically significant disease. Lesions are light brown and mostly round, with dark brown margins.
Chimera-also known as a somatic mutation, affected leaves often lack pigment in mottled patterns. Since the mutation occurs in portions of the plant after germination, the patterns may affect only a portion of the plant. This is not an infectious problem and rarely causes any economic loss. Somatic mutations also occur in other crops.
Cross stitch-This is an unexplained disorder. It is characterized by crack-like lesions that run perpendicular to the sutures of watermelon. This disorder is not a common one and is usually not economically important. It is apparently non-infectious.
Damping-off of watermelon-symptoms of damping-off include the collapse and wilt of affected seedlings. A brown necrosis can often be observed at the soil level.
Dodder-dodder is a parasitic plant that can affect watermelon as well as many other plants. The plant lacks chlorophyll and thus appears yellowish. Not common or important in Indiana.
Downy mildew-This disease does not occur every year in Indiana and watermelon may be out of production by the time the fungus-like-organisms arrives in Indiana. Initial symptoms on leaves are often light chlorotic lesions with diffuse margins. Lesions on the underside of leaves may have a black or purple sporulation under moist conditions. The center of lesions may turn necrotic with time. Stems and fruit are not directly affected.
Fusarium wilt-An important disease and limiting factor in watermelon production in Indiana. Often the initial symptom that is observed is a one-sided wilt. Similarly, a portion of the vascular system may exhibit discoloration. Symptomatic plants are often clustered in the field.
Gummy stem blight of watermelon
Gummy stem blight-One of the most common foliar diseases of watermelon in Indiana. Lesions on leaves are amorphous, dark and may appear water-soaked. Dark fungal bodies (pycnidia) may be observed in lesions on leaves or stems. I have never observed fruit infection of watermelon in Indiana.
Lightning damage of watermelon
Lightning damage-This is not an infectious problem and it not economically important. However, it is important to recognize lightning damage and distinguish it from other problems. Symptoms vary, but affected areas in fields may be roughly round, but do not expand over time. Affected plants may be rotted or even appear scorched. Note that nearby weeds are also affected.
Manganese toxicity of watermelon
Manganese toxicity-This is not an infectious disorder. However, it is important to recognize the symptoms since economic loss can occur. Also, the symptoms tend to be clustered in areas of the field where the soil pH is relatively low, thus mimicking an infectious problem.
Phytophthora blight of watermelon
Phytophthora blight-Perhaps the most important disease of watermelon in Indiana. Generally, leaves and stems are not affected (compare with pumpkin). Fruit become soft, water-soaked and a white mold will develop on the fruit.
Powdery mildew-not as important or common a problem in Indiana as it is in the southeast US. Symptoms are easily recognizable from the white talc-like symptoms of lesions.
Root knot nematode of watermelon
Root-knot nematode-Often the first evidence of this disease is the wilt and decline of the host plant. When the roots are sampled, the galls caused by the nematode are evident. Can be an important disease and is relatively widespread. Probably too often overlooked.
Rind necrosis-Occurs only sporadically and is not usually an important problem. Not an infectious disease. Affected watermelon may feel ‘knobby’ on the surface before being cut. Lesions inside the rind consist of a brown necrosis and may appear rotten. May affect marketability.
Target cluster-This disorder is not well understood. It is apparently not an infectious disorder. However, in two instances documented below, the symptoms seem to be associated with a potyvirus infection. Target cluster is not a common or economically important disorder.
Virus-The most common virus diseases of watermelon in Indiana are potyviruses. Watermelon do not have viral symptoms as much as pumpkins since the latter are grown later in the season. Symptoms on foliage may include mosaic and shoestring leaves. Symptoms on fruit are not common. Viruses do not usually cause economic damage to watermelon.
White mold-While this disease has a huge host range, I have not known it to be an important problem on watermelon in Indiana. Below it is shown only in a transplant tray. However, I have observed vine death in the field due to white mold. Diagnostic is the irregular, dark sclerotia as in other crops.
Zucchinis are often grown alongside summer squash for the retail market. Often planted at intervals throughout the spring and early summer to maintain a constant source of the fruit. A common crop, but not usually grown in large acreages in Indiana.
Powdery mildew-This disease is easily recognized by the white, talc-like appearance of lesions on leaves and stems. Powdery mildew will usually not be severe until plants have been established for some time. Since zucchini plants are often continually planted, by the time the disease is severe on old plants, younger plants are in production.
Silver leaf-this symptom is caused by the feeding of the silver leaf whitefly. This whitefly does not overwinter in Indiana. Thus, this symptom is not common and will usually not occur until late in the season. The proper identification of the silver leaf whitefly is critical in the diagnosis of this problem. The silver leaf whitefly may feed on several vegetable crops.
White mold-This disease can be recognized by the dark, irregularly shaped fungal bodies known as sclerotia that can be found inside or outside the stem or associated with fruit. In addition, a white mold normally accompanies the sclerotia.