The fungus spends the winter in infected plant debris in or on the soil where it can survive for several years. Infections are most prevalent on poorly nourished or otherwise stressed plants. Lesions may form within 2-3 days or the infection can remain dormant awaiting proper conditions (60 degrees F and extended periods of wetness, such as provided by rain, mist, fog, dew, irrigation). Infected leaves eventually wither, die, and fall from the plant. The leaf blight phase usually begins on the lower, older leaves and progresses up the plant. Dark, concentric circles can be seen within these lesions. The classic symptoms occur on the leaves where circular lesions up to 1/2" in diameter are produced. This disease can cause a major loss by infecting the fruits and indirect losses by reducing plant vigor.Įarly Blight produces a wide range of symptoms at all stages of plant growth: the most common are stem cankers, leaf spots, and fruit rot. It is important to use fungicide chemistries with different modes of action in the spray program to prevent fungicide resistance build-up.A common problem in many home gardens is a plant disease called Early Blight of tomato, caused by the fungus Alternaria solani. There are reports of certain fungicides losing their efficacy on many isolates of the early blight pathogen. Fungicide applications should begin just after bloom or at the first sign of infection, whichever is earlier. Consult with provincial agricultural representatives for current fungicide recommendations. Fungicides are important in suppressing damage caused by the early blight fungus.The early variety can act as a source of infection for later varieties in adjacent fields. Early maturing cultivars are often very susceptible to early blight. Highly susceptible cultivars should be avoided where losses to early blight are expected. No truly resistant cultivars are available. Control with good fertility, foliar fungicides, and by reducing plant stress.Early blight is often more severe when the potato crop has been under stress, ie., poor nutrition, injury, insect damage, or drought. Note that the fungus can often be recovered from healthy-looking leaf tissue as young leaves and crops heavily fertilized with nitrogen do not generally exhibit symptoms.īecause the fungus overwinters in the field on dead leaves and vines or on infected tubers, early blight will initially be found along margins of fields that border those planted to potatoes the previous season. Look for small, dark-brown to black spots on older leaflets on the lower portions of plants. This disease develops best when the leaves of adjacent rows touch because this closeness provides optimum conditions of high humidity, leaf yellowing of lower leaves, and moderate canopy temperatures. Inspect fields after the plants reach 30 cm (12 in.) in height to detect early infections. The underlying tissue shows a brown, corky, dry rot more than 2.5 cm (1 in.) deep. They also often have a characteristic `target' appearance because of the concentric rings of raised and depressed necrotic tissue.Īlthough rare, tuber infection may occur resulting in irregular, brown-black, sunken spots, surrounded by a raised violet border. Lesions first appear on the older, lower leaves, spreading to the younger leaves under favourable weather conditions.Īdvanced lesions often have angular margins as they move up to and getting restricted by major leaf veins. The first symptoms are small spots (dry and papery in texture) that become oval to angular, dark-brown to black spots as they expand. Tubers may become infected when they are lifted through soil infested with spores that are concentrated at ground level. Lesions may also develop on stems, but it is rare for significant injury to occur from this infection. Secondary infections occur when spores from primary lesions are spread to healthy leaves. The spots may enlarge and coalesce, causing the leaflets to die. A narrow light green to yellow halo frequently surrounds the spot. irregular, concentric rings in the dead tissue. The spots on the lower leaves enlarge and develop a characteristic 'target spot' i.e. Alternating wet and dry conditions are the most favorable for spore production and spread. Spores may be carried to leaf surfaces by splashing water or wind. Mycelium and spores overwinter in soil or on crop residue (dead leaves and vines, infected tubers). It is most severe on maturing foliage and plants or under-fertilized potato vines. Under severe infections, leaves are often killed, reducing yields, and tuber infections make the diseased potatoes unmarketable. The disease commonly occurs in most potato growing areas. Unlike the name suggests, the disease often first appears in late July.
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