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Vegetable diseases enhanced by rain

Growers and pest control advisors know that rain and free moisture are required for many plant pathogens to survive, infect plants, and cause disease and crop losses.

Due to the winter and spring rains of 2009-2010, especially the late February through April storms, growers are seeing diseases that mostly develop in years with rainy spring periods. Other very common diseases, including downy mildews, are getting an early start at infecting crops.

Below are summaries and descriptions of several of these problems. For additional details on these and other diseases, read the book, Vegetable Diseases: A Color Handbook by Steven Koike, Peter Gladders, and Albert Paulus.

• Lettuce: Anthracnose

Early symptoms are less than one-quarter inch wide, water-soaked leaf spots. Spots enlarge, turn tan, and are usually angular in shape. White to pink spore masses are visible in the center of the tan spots.

If the disease is severe, the lesions merge together and cause leaf dieback and plant stunting. As spots age, the affected tissue dries up, becomes papery in texture, and may tear. Anthracnose lesions are often clustered along the midribs of the lower leaves.

Control measures: Preventative fungicides and avoid planting lettuce in fields with a history of anthracnose.

• Lettuce: Bacterial leaf spot

Early symptoms are small (one-eighth to one-quarter inch), water-soaked spots that occur on the outer leaves of the plant. Lesions are typically angular in shape. Lesions quickly turn black. This is the diagnostic feature of the disease.

If the disease is severe, numerous lesions can coalesce resulting in leaf collapse. With time, lesions dry up and become papery in texture yet retain the black color. The pathogen rarely infects the inner younger leaves.

Control measures: Since the pathogen can be seedborne, use clean seed. Also avoid planting back-to-back lettuce if the first crop was diseased.

• Lettuce: Downy mildew

Downy mildew results in light green to yellow angular spots on the upper surfaces of leaves. White fluffy growth of the pathogen develops mostly on the undersides of the spots. The lesions eventually turn brown and dry up.

Control: Resistant cultivars and preventative fungicides.

• Spinach: Downy mildew

Like lettuce downy mildew, this is a very familiar and readily identified disease which causes light green to yellow spots on leaves. The characteristic purple sporulation occurs mostly on the underside of leaves. Multiple new races are causing significant concern for growers.

Control measures: Resistant cultivars and preventative fungicides.

• Spinach: Cladosporium leaf spot

In California, this disease rarely occurs without a rainy spring. Symptoms include round to oval, tan leaf spots which do not exceed one-quarter inch wide. Dark green spores and mycelium develop in the center of the spots and are characteristic signs of the pathogen.

Control: None recommended.

• Cilantro: Bacterial leaf spot

This disease causes angular, dark brown to black spots to occur on the cilantro leaves. The disease can be found on younger and older foliage. Once the disease starts in one part of the bed or field, splashing water from rains rapidly spreads the pathogen to surrounding plants.

Control measures: Since the pathogen can be seedborne, use clean seed. Bactericides are of limited use. Avoid sprinkler irrigation if possible.

• Broccoli: Head rot diseases

Rain and foggy weather have caused significant losses from head rot this year. Bacterial and Alternaria head rots result in the water-soaked or greasy discoloration of small groups of unopened flowers.

Later, the affected portions of the head turn brown to black and the infection spreads and affects larger parts of the head. The tissue turns soft as the pathogens and secondary decay organisms rot the head.

Control: Select cultivars less likely to develop the disease.

Garlic and leek: Rust

Initial symptoms of rust on garlic and leek consist of less than one-eighth inch long, elongated white flecks on leaves. Flecks enlarge into raised leaf pustules. The leaf tissue covering the pustule breaks open and masses of brown orange spores become visible. A dark brown to black spore stage may be seen later in the season.

Extensive pustule development causes leaves to dry up and die. Garlic rust and leek rust are caused by two different rust pathogens.

Control: Preventative fungicides.

• Fennel: Cercosporidium blight

This disease primarily affects older foliage. Affected leaf tips and stems turn brown to black in color and dry up. Examination of the stems and leaves show tiny, discrete, dark brown to black fungal patches. Early patches are less than one-sixteenth inch wide, and can be oval, circular or irregular in shape.

If the disease is severe, the patches multiply and grow together resulting in an overall dark appearance and death of the foliage. If sufficient humidity and moisture occurs, a white crusty growth forms on top of the patches. The white crust is made up of clusters of the pathogen’s spores. Cercosporidium blight does not kill fennel plants, but can impact growth and result in a poor quality product.

Control: None recommended.

• Artichoke: Ramularia leaf and bract spot

Initial symptoms consist of less than one-quarter inch wide pale-to-yellow green circular spots. The spots can expand up to one-half inch wide and turn brown. Spots are visible from the upper and lower surfaces of leaves.

If the disease is severe, lesions will coalesce and the entire leaf can turn brown and dry up. White growth of the fungus usually develops in the center of leaf lesions. Ramularia leaf spot is economically important when the pathogen moves from the leaves to the flower bud bracts. On the bracts, brown, irregularly shaped, patchy lesions form causing the bracts to curl, split and dry out.

Control: Preventative fungicides.

• Strawberry: Leaf blotch

Symptoms consist of tan-to-gray leaf lesions that develop on the first few leaves of the growing transplant. The infected areas grow fairly large covering one-quarter to one-half of the leaflet surface.

Leaf infections commonly grow from the margin or edge of leaflets, are irregular in shape, and can be surrounded by a purple red border. An important sign of leaf blotch is the presence of tiny brown-to-black colored fungal fruiting bodies in the gray blotches.

Control: None recommended.

• Strawberry: Angular leaf spot

Initial symptoms consist of water-soaked spots on leaves. The spots enlarge to form translucent, angular lesions bordered by leaf veins. As the disease progresses, lesions turn into reddish brown spots which later become necrotic and can merge together, resulting in the death of leaf sections.

Under humid conditions, the spots ooze a cloudy film of bacteria that dries into a crystalline layer.

Control: None recommended.

TAGS: Management
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