Germination blotters are cut to fit and placed in the bottom of each box. The blotters are moistened with sterile deionized water. Pathogen Identification 1. The boxes are read by scanning them with a binocular stereoscope at 7x power. Infested seed may show a soft brown rot and are covered with a fine mass of mycelium with large sword shaped conidia protruding from the mass. Transfer suspect spores to a microscope slide and confirm spore morphology at x power.
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Infection can be initiated by seedborne or windborne spores conidia , which germinate in the presence of free moisture and penetrate leaves through wounds or natural openings. Spores conidia are readily disseminated by wind and splashing water in and among adjacent fields; large numbers of conidia become airborne during harvesting. The pathogen can be moved among fields on contaminated equipment. Plant Response and Damage Alternaria leaf spot symptoms first appear as greenish brown, water-soaked lesions.
Lesions quickly become dark brown to black with or without yellow halos. Under disease-favorable conditions, lesions coalesce and cause entire leaves to become yellow, collapse, and die. Older leaves are most susceptible to infection and often the first to develop symptoms, but all leaves can be infected.
Petiole lesions appear similar to leaf lesions, but are more elongated; petiole lesions quickly kill entire leaves. Disease symptoms are often confused with Cercospora leaf spot and bacterial blight, and microscopic examination is often necessary for accurate diagnosis.
Carrot seedlings can also be attacked by A. Infection occurs at or just below the soil line. Infected hypocotyls and upper roots become girdled and shrivel, turning a gray or black color. Seedling infection appears similar to Pythium damping-off, but A. Management Approaches No biological control practices have been developed for Alternaria leaf spot. Cultural Control Plant high quality seed free of the Alternaria leaf spot pathogen. Hot water treatment can reduce seed contamination, but may reduce seed storability and germination.
Practice a three-year or longer crop rotation to nonhosts such as small grains. Eliminate potential sources of the pathogen by deeply burying crop debris after harvest and controlling wild and volunteer carrot. Avoid prolonged periods of leaf wetness by avoiding dense planting, orientating rows parallel to the prevailing wind direction, and timing irrigations to end before dusk.
Avoid overhead irrigation if possible. Chemical Control Fungicides are rarely necessary for Alternaria leaf spot control in many High Plains carrot production regions, but are essential in warm, humid environments.
Disease forecast models have been developed that can improve the timing and efficiency of sprays. Chemical controls are most effective when combined with as many cultural control strategies as possible.
Alternaria Leaf Spot