. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. PERIOD OF INCUBATION. 65 The time between inoculation and visible disease may be as short as 24 to 48 hours, or as long as 3 or 4 weeks. It varies not only with different organisms, but with the same organism under different conditions. In some species long cultivation on artificial media destroys or greatly weakens the ability of the organism to attack tissues. In other cases a similar reduction of virulence occurs within the host. Experimenting with juicy suscepti- ble plants and such organisms as Bacillus carotovorits,
. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. PERIOD OF INCUBATION. 65 The time between inoculation and visible disease may be as short as 24 to 48 hours, or as long as 3 or 4 weeks. It varies not only with different organisms, but with the same organism under different conditions. In some species long cultivation on artificial media destroys or greatly weakens the ability of the organism to attack tissues. In other cases a similar reduction of virulence occurs within the host. Experimenting with juicy suscepti- ble plants and such organisms as Bacillus carotovorits, B. olcraccac, B. aroideae, B. fiiclo)iis, or B. hyaciiitlii (lieinz), the result of a single needle-prick is often visible in 24 hours, and by the end of the third day the necrosis of tissue is often quite extensive. With the same organisms and in the same host-plants, but in rather woody or somewhat dry spongy tissues, the progress of the disease is slow, and after a slight development it may stop altogether. Potter states that his Ps. destniciaiis inoculated into turnips caused very distinct signs of the disease in 24 hours. For their rapid development most of the soft-rot organisms require tissues full of water. With Bacillus pliytophthorus, using virulent cultures, suscepti1)le varieties of potatoes, and optimum temperatures, and inoculat- ing by needle-pricks, rot is always visible in 24 to 48 hours, and the entire tuber may be rotted in a week's time, even in dry air. In pear-blight the blackening of the shoots usually occurs in from 3 to 10 days after inoculation by needle-punctures from fresh agar cultures (vol. I, plate 28), but may sometimes be delayed 23 days (Arthur). Much depends on the weather and on the immaturity of the shoots. The pear-blight develops soonest in moist, warm weather and in rapidly growing shoots. In blossom- infections there is a distinct browning in the nectaries in 48 hours, and on the third or fourth day the whole flower collapses and is blac
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