Use pathogen-free seed, or collect seed only from disease-free plants.Spores can be spread throughout a field by wind, human contact or equipment, resulting in many reinfection opportunities throughout a growing season.Spores infect plants and form leaf spots as small as 1/8 inch diameter in as little as five days.Spores (reproductive structures) can germinate between 47° and 90° F and need free water or relative humidity of 90% or greater.Lower leaves become infected when they come into contact with contaminated soil, either through direct contact or when raindrops splash soil onto the leaves.The pathogen also survives on tomato seed or may be introduced on tomato transplants. The early blight pathogens both overwinter in infected plant debris and soil in Minnesota.The pathogen is most likely to spread with any weather or heavy dew, or when relative humidity is 90% or greater.Disease develops at moderate to warm (59 to 80 F) temperatures 82 to 86 F is its optimum temperature range.Both pathogens can infect tomatoes, potatoes, peppers, and several weeds in the Solanaceae family including black nightshade ( Solanum ptycanthum), and hairy nightshade ( Solanum physalifolium).Early blight can be caused by two closely related species: Alternaria tomatophila and Alternaria solani.
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