Fungi of the Alternaria genus are widespread nearly in all climatic zones, they can lead saprophytic mode of life and also cause diseases of numerous agricultural cultures. High adaptive potential of the fungus is conditioned by it having wide spectrum of ferments and toxins that make it possible to occupy and develop various living and dead substrates, while copious sporification and most diverse ways of conidia spreading make it omnipresent.
Citrus plants, especially oranges, are affected by the fungi Alternaria citri Ell. et Pierce. On the surface the fruits look healthy, only sometimes in the place of attachment of the fruit stem or on one side of the fruit a small brown spot appears. On cutting the fruit from the top one discovers Alternaria black rot (fig. 1, 2). The pathogen of the disease spreads in the tissues of the axile plane of the fruit. The infected tissues darken, soften, their surfaces reveal soft thin coating of just-formed mycelium (fig. 3, 4). Downy mycelium active development begins already within a few hours after having the fruit cut. First it is white in color, than darkens quickly as the conidia and conidiophores are formed (fig. 5, 6, 7, 8,). Conidiophores are simple or branchy, straight or sinuous, with septa, of different shades of brown, up to 300x5 µm. Conidia are single, in simple or branchy chains (fig. 9, 10, 11, 12, 13, 14), prolonged, egg-shaped, inversely-clavate shaped, oblong-ellipsoidal, light-, dark-, or olive-brown, smooth or verruculose, with several cross or long septa, with a lighter cervix or without it, 8-60 (42) x 6-24 (17) µm in size. The conidia can form immediately on the mycelium, on the conidiophores or straight on the conidia (fig. 19, 20, 21, 22). Under favorable conditions conidia sprout giving rise to vegetative mycelium growth, or alternatively form up new conidia (fig. 23, 24, 25, 26).
Most frequently the disease affects stored fruits, especially those that have hypovitability due to the long storage. Oranges can be affected on the plantations as well. The development of the disease in the storehouses slows down under the lower temperatures and moderate air moisture.
|All images by G. Pestsov.|
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