Alternaria Blotch has been a serious problem in the orchards of Himachal Pradesh. Alternaria leaf blotch is a fungal disease which attacks leaves of apple trees, causing lesions to form on the leaves during late spring or early summer. Alternaria infections begin with small, dark, circular spots which are about ½ inch in diameter. Alternaria spots on spreading change in color from black to grey with outside yellow circle. It is prominent in orchards with heavy mite infestations. So maintaining mite is an important factor in preventing alternaria blotch.
The fungus of alternaria blotch overwinters on dead leaves on the orchard floor, in twigs, in mechanical injuries or in dormant spurs. Infection starts firstly about one month after petal fall. Secondary infections occur throughout the summer in hot and rainy weather. The infection occurs in 5-10 hours at optimum temperatures. Severe infections result in up to 50% defoliation of trees hence reducing fruit yield and quality.
Primary symptoms of alternaria blotch are spots on the leaves, which appear in late spring and early summer. Initially these spots are about ¼ inch to ½ inch in diameter and brown in color. At later stage these spots can become irregular in shape and can become grayish in color. When the leaves are heavily infested by the fungus, defoliation of the tree can occur.
Destroy apple leaves to keep the Alternaria leaf blotch fungi and mite eggs from overwintering in the debris. Removing all weeds and raking up and discarding fallen plant litter also helps reduce the risk of the disease reoccurring the following growing season. Mulch can also help to slow the spread of alternaria spores already in the soil when applied immediately after planting. Sanitation plays key role in controlling Alternaria Blotch.
Strobilurin group fungicides are registered for management of Alternaria blotch. It is very important to maintain mites below 5 per leaf in orchards, in order to minimize defoliation. Chemicals found effective against Alternaria Blotch are——