عارضه لکه سیاه سیب

Symptoms and Signs Apple scab results in symptoms on the aerial parts of the apple tree, including leaves, petioles, flowers, sepals, fruit, pedicels, young shoots, and bud scales. Apple leaves and fruit The symptoms are generally most noticeable and serious on leaves and fruit. The first lesions seen in the spring are usually on the underside of expanding leaves. Once the leaves open, the upper surfaces also become vulnerable to infection. A lesion first appears as an area which is a lighter shade of green than the surrounding leaf. The lesion is usually circular and as it increases in size it becomes olive-colored and velvety due to production of asexual spores (conidia) (Figure 2). Lesions that form on young leaves may be quite large, some more than 1 cm in diameter. Lesions that form on expanded leaves are usually smaller because older leaves are more resistant to infection. Affected tissues eventually may become distorted and puckered, and the leaf lesions often become cracked and torn. Lesions on the leaves and fruit are generally blistered and "scabby" in appearance, with a distinct margin (Figures 3 and 4). The earliest noticeable symptom on fruit is water-soaked areas which develop into velvety, green to olive-brown lesions. Infections of young fruit will cause fruit distortion (Figure 5). Severely infected leaves or fruit will often drop from the tree. Infection which causes significant defoliation for two or three years in a row can result in weakened trees that are more susceptible to freeze damage, insect injury, and other diseases. Figure 2 Figure 3 Figure 4 Figure 5 Apple blossoms Symptoms on blossoms usually occur as small, dark green lesions at the base of the flower, on the sepals, and on the stem pedicel before and during bloom (Figure 6). When pedicels become infected the developing fruit may drop, resulting in lower fruit yield.