农病全部英语部分.docVIP

APPLE SCAB Apple scab exists worldwide but is more severe in areas with cool, moist springs and summers. Similar scab diseases affect pears (Venturia pyrina) and hawthorns (V. inaequalis sp.f. pyracanthae). Scab is the most important disease of apples. Its primary effect is reduction of the quality of infected fruit. Scab also reduces fruit size or results in premature fruit drop, defoliation, and poor fruit bud development for the next year;and it reduces the length of time infected fruit can be kept in storage. Losses from apple scab may be 70% or more of the total fruit value. In most apple-producing areas, no marketable fruit can be harvested if scab control measures are not taken. Symptoms At first, light, olive-colored, irregular spots appear on the lower surface of sepals（萼片） or young leaves of the flower buds. Soon after, the lesions become olive green gray with a velvety surface. Later, the lesion; appear metallic（金属的） black in color and may be slightly raised. Lesion on older leaves generally form on the upper surface of the leaves. Lesions may remain distinct or they may coalesce（联合）. Leaves infected young remain small and curled and may later fall off. Occasionally,small scab spots are produced on twigs and blossoms. Infected fruit develop circular scab lesions, velvety and olive green at first but later becoming darker, scabby, and sometimes cracked. The cuticle of the fruit is ruptured（破裂、裂开） at the margin of the lesions. Fruit Infected early become misshapen（畸形） and cracked, and frequently drop prematurely . Fruit infected when approaching maturity form only small lesions, which, however, may develop into dark scab spots during storage. The Pathogen: Venturia inaequalis The mycelium in living tissues is located only between the cuticle and the epidermal cells. There, it produces short, erect, brownish conidiophores that give rise to several, one- or two-celled, and spilocaea-type(环痕式 ) conidia of rather characteristic shape. In dead leave