Plant Diseases

Potato Early Blight

potato leaf showing lesions
Potato early blight lesions. Photo Credit: Howard F. Schwartz, Colorado State University, via Bugwood.org

Early blight is a fungal disease of Solanaceous crops caused by Alternaria solani. It causes circular brown spots on leaves and stems that can later develop concentric, target-like rings, often surrounded by yellow margins. Lesions are sometimes limited by veins giving an ‘angular’ appearance.  The pathogen causing early blight survives on infected soil-bound plant debris and spreads via air and water splashing to new plant tissues. Favorable conditions include poor airflow, high moisture, and nitrogen deficiency and maturity in plants. Cultural management involves maintaining proper moisture, airflow, and nitrogen levels, as well as rotating away from susceptible crops. Little is known about tomato early blight resistance. Most commercial potato varieties are susceptible to early blight. Well-timed fungicide applications can also manage disease progression.

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Potato Late Blight

Typical late blight lesion on the top of a potato leaf. It shows a circular to irregularly-shaped brown, papery lesion typical at 7-10 days old in relatively dry weather.

Late blight is a disease that typically causes severe symptoms on flowering plants of the family Solanaceae. The pathogen that causes late blight, Phytophthora infestans, infects a variety of Solanum spp., including eggplant, pepper, nightshades, and petunia. However, it is most infamously known for its destruction of potato, S. tuberosum, and tomato, S. lycopersicum.

Symptoms of late blight can occur on all parts of a potato plant. Leaf symptoms include circular, necrotic or brown lesions surrounded by collapsed pale or chlorotic (pale green to yellow) tissue. Enlarged, water- soaked or wet and oily-appearing leaf lesions often give rise to sporulation, identifiable by white or gray fuzzy growth. Dark green, brown, or black water-soaked lesions on the stem may also contain sporulation. Symptomatic tubers typically have sunken and firm brown lesions that may extend several centimeters into the tuber. The variability in lesion appearance is often the result of differences in moisture.

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Potato Silver Scurf

Silver scurf lesions on a potato tuber
Silver scurf lesions on a potato tuber. Photo Credit: Amanda J. Gevens, UW Extension.

Silver scurf is a fungal blemish disease of potato tubers caused by Helminthosporium solani. The pathogen causes tan-to-gray circular lesions typically initiating on the stem-end of the tuber surface, often appearing shiny and silver when wet. The pathogen survives on infected tubers and infested plant debris, spreading via rain/irrigation in the field and airborne spores in storage. Favorable conditions include high temperatures and humidity. Infection reduces both the visual appeal of a fresh market potato tuber and reduces the quality of potato tubers as the infection creates damage to the periderm or skin which enables the onset of other pathogens or enhanced desiccation.  Cultural management involves using clean, uninfected seed potatoes, rotating planting sites, promptly harvesting fields after potato foliar senescence, disinfecting storage facilities, and maintaining properly cold and ventilated storage areas. Seed-applied, in-furrow, and post-harvest fungicides can also reduce disease incidence. This disease is primarily a concern for stored, commercial fresh market potatoes.

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Potato Brown Spot and Black Pit

Potato leaf showing brown spot and black pit lesions
Potato leaf showing brown spot and black pit lesions. Photo Credit: Howard F. Schwartz, Colorado State University, via Bugwood.org

Brown spot and Black pit are fungal diseases of potato caused by the fungus Alternaria alternata. On leaves, it causes dark brown spots of necrotic tissue with a dark brown margin. Starting as small lesions, the spots can coalesce to cover a large percentage of leaf or petiole surface. On tubers, the disease causes black, deep sunken pits with definite margins that often develop during in storage. The pathogen causing these diseases survives on infected soil-bound plant debris and susceptible weeds, and spreads to leaves via windborne spores and tubers through soil-bound spores, often after mechanical damage. Favorable conditions include long dew periods, standing water on foliage, and warm or hot temperatures. Reduced airflow, plant maturity, and low nitrogen status of a potato crop can also favor these diseases. Cultural management involves properly managing soil moisture, post-harvest plant debris, crop fertilization, rotating away from Solanaceous crops, and avoiding bruising tubers during harvest. Properly-timed foliar fungicide applications can also aid in managing the disease.

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Potato Pink Rot

Exterior and interior tuber symptoms of potato pink rot. Photo Credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, via Bugwood.org

Pink rot is a water mold or oomycete disease of potato tubers caused by Phytophthora erythroseptica. The soil-borne pathogen causes darkened, water-soaked lesions with defined margins near the stem-end of the tuber, and is often identified by the characteristic pink/salmon color and ammonia-like odor of a potato cut and exposed to air after 20-30 minutes. The pathogen survives in infested soil, and infects tubers in the field either through direct contact with soil or swimming zoospores during wet conditions. During harvest and in storage, disease can spread between tubers through bruises and wounds. Favorable conditions include high temperatures, high moisture, and poorly draining soils. Cultural management involves removing plant debris and volunteer potatoes post-harvest, managing soil moisture, scouting poorly-draining areas for pink rot pre-harvest, avoiding harvesting in hot temperatures and storing tubers in cool, well-ventilated facilities. Pre-harvest chemical management can also prevent disease development.

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Tomato Early Blight

Tomato leaf showing symptoms of early blight. Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, via Bugwood.org

Early blight is a fungal disease of Solanaceous crops caused by Alternaria solani. It causes circular, dark brown lesions with concentric, target-like rings. These foliar lesions often first develop on older, more mature foliage, and are commonly surrounded by yellow margins. Similar lesions can also develop on stems and fruit, with infected tomato fruit often dropping off of the vine. The pathogen causing this disease survives on infected soil-bound plant debris, seeds, and diseased plant tissue, spreading via windborne or water-splashed spores. Favorable conditions include poor airflow, high humidity, and high leaf wetness. Cultural management involves using pathogen-free seed, fumigated or pathogen-free soil, planting resistant varieties if available, rotating fields for at least two years, destroying host weeds and volunteer plants, and properly fertilizing fields. Fungicides can also provide good control of early blight in vegetables when applied early on in infection. Multiple applications of fungicide are often necessary to sustain disease management to time of harvest.

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Tomato Late Blight

Whole tomato plants defoliated due to late blight infection. Note fruit hanging on plants despite loss of vines and stems.

Late blight is a disease that typically causes severe symptoms on flowering plants of the family Solanaceae. The pathogen that causes late blight, Phytophthora infestans, infects a variety of Solanum spp., including eggplant, pepper, nightshades, and petunia. However, it is most infamously known for its destruction of potato, S. tuberosum, and tomato, S. lycopersicum.

Symptoms of late blight can occur on all parts of a potato plant. Leaf symptoms include circular, necrotic or brown lesions surrounded by collapsed pale or chlorotic (pale green to yellow) tissue. Enlarged, water- soaked or wet and oily-appearing leaf lesions often give rise to sporulation, identifiable by white or gray fuzzy growth. Dark green, brown, or black water-soaked lesions on the stem may also contain sporulation. Symptomatic tubers typically have sunken and firm brown lesions that may extend several centimeters into the tuber. The variability in lesion appearance is often the result of differences in moisture.

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Tomato Septoria Leaf Spot

Tomato leaf showing symptoms of Septoria leaf spot. Photo credit: Bruce Watt, University of Maine, via Bugwood.org

Septoria leaf spot is a fungal disease of solanaceous crops caused by Septoria lycopersici. Symptoms appear on the leaves as circular, tan-to-gray spots with darker brown margins and dotted with dark, raised pycnidia inside the tan portion of the lesion. These lesions are often surrounded by a yellow halo, and typically develop on lower leaves first. Stem lesions appear similar to leaf lesions, but are often darker, and fruit lesions appear similar but are uncommon. The pathogen causing this disease overwinters on diseased Solanaceous crop or weed debris in the soil, and spreads to older leaves via splashing water, workers, equipment, and several insects. The pathogen then spreads to newer leaves from resulting lesions in a similar way. Favorable conditions include poor airflow, high moisture, and moderate temperatures. Cultural management involves planting resistant varieties, rotating away from susceptible crops, staking or trellising plants, destroying host weeds and infested plant debris, avoiding over-irrigating, and properly mulching. Preventative application of conventional or organic fungicides can also prevent disease development.

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Cucurbit Alternaria Leaf Blight

Cucurbit (squash, cucumber, melon, etc) leaf showing symptoms of Alternaria leaf blight. Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, via Bugwood.org

Alternaria leaf blight is a fungal disease of cucurbits caused by Alternaria cucumerina. Cucurbit crops include watermelon, cantaloupe, cucumber, zucchini, pumpkin, and winter squashes. This pathogen causes brown lesions on the leaves that can develop concentric, target-like rings and a yellow surrounding halo. Fruit can experience sunscald from leaf loss, as well as reduced yield. A. cucumerina survives on infected soil-bound plant debris, spreading via wind, splashing water, workers, equipment, or insects. Favorable conditions include warm temperatures and high moisture. Cultural management options include rotating away from susceptible crops, maximizing distances between susceptible cucurbit fields, avoiding overhead irrigation and working in wet conditions, and destroying or tilling under infected crop residue. Appropriately-timed fungicide applications can also prevent disease progression.

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Cucurbit Downy Mildew

Cucurbit (squash, cucumber, melon, etc) leaf showing symptoms of Cucurbit downy mildew. Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, via Bugwood.org

Cucurbit downy mildew is a water mold or oomycete disease of cucurbit crops caused by Pseudoperonospora cubensis. Symptoms first appear on the upper leaf surface as angular, vein-bounded, yellow to pale-green spots, turning brown and coalescing to turn entire leaves brown with disease progression. In very humid conditions, the underside of leaves may appear fuzzy as the pathogen produces numerous spores which enable the pathogen to spread. The disease survives on living cucurbit plant tissue, and spores spread via wind, rain and irrigation splash, or human spread on equipment and hands. Favorable conditions include high humidity and leaf wetness, as well as moderate temperatures. Cultural management involves planting resistant cultivars, avoiding overhead irrigation, and practicing wide row spacing to improve airflow.  Knowledge of the cucurbit plant types affected can be very useful as there are now two known strain types, one which infects exclusively cucumbers and cantaloupes, and one which infects pumpkins, winter squash, and watermelons. The cucumber and cantaloupe strain type tends to have more fungicide resistance and aggressiveness.

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Carrot Alternaria and Cercospora Leaf Blights

Alternaria and Cercospora may be found on the same plant and are often indistinguishable from each other.

Alternaria and Cercospora leaf blights are common fungal diseases of carrot leaves and petioles. While neither disease directly hurts the carrot root, yield loss occurs when petioles become so brittle that they break off during mechanical harvest leaving the carrot in the ground. In warm, moist weather, Alternaria leaf spots develop rapidly so that the entire field may appear to have been injured by frost or chemicals. Crop loss will be more severe when plants become infected early in the season.

Alternaria and Cercospora are difficult to distinguish in the field and often appear on the same plant. The principle difference involves disease timing. Cercospora leaf blight, caused by the fungus Cercospora carotae, attacks young rapidly growing plants; Alternaria leaf blight, caused by the fungus Alternaria dauci, primarily attacks older plants, although seedlings may also be infected.

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Onion Botrytis Leaf Blight/Leaf Spot

Onion leaf showing Botrytis leaf spot symptoms. Note the small, whitish, oval-shaped spots on the leaf surrounded by a light green or silver halo. Photo credit: Lindsey du Toit, Washington State University, via Bugwood.org

Onion Botrytis leaf blight/leaf spot is a fungal disease of alliums caused by Botrytis squamosa. Symptoms first appear as small whitish spots on the leaf. These spots are oval-shaped, and sometimes surrounded by a light green or silver halo that often appears water-soaked. Leaf tips will begin to dry and wither as the disease progresses, sometimes until the whole leaf dies back. Progressed infection can stunt bulb growth and reduce yield. Heavily infected fields often appear yellowish and blighted. Severe infection can stunt bulb growth and reduce yield.

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