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 carotovorus, B. olcraceae, B. aroideae, B. melonis, 

 or B. hyacinthi (Heinz) , 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. dcstructaus 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 phytophthorus, using virulent cultures, susceptible 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 blackened, together with its pedicel, 

 which has become infected. In the writer's 

 experiments with Bacterium solanacearum 

 in 1895 and 1896, blight appeared in young 

 shoots of the potato and tomato in about 

 4 to 6 days when they were inoculated by 

 needle-pricks from young cultures. On the 

 contrary, in an old and woody tomato plant 

 wilt did not become general until 8 or 9 

 weeks after the punctures, but then the 

 organism was found to have multiplied enormously in the vascular system, extending to a 

 distance of several feet from the pricked part of the stem. In large tomato plants in a field 

 in South Carolina, during wet weather in July 1895, direct infections by needle-stab induced 

 plain signs of the disease only after many days. Similar tardy results were obtained in 

 Washington in a hothouse in 1909. When Colorado potato beetles were used as carriers of 

 the organism the first signs of the disease in potato appeared in 7 to 9 days from the time 

 the plants were bitten. In more recent experiments with this organism, especially some 

 tests made in 1904 with an extremely virulent strain obtained from a blighting potato, wilt 

 appeared in young tomatoes in 48 hours after inoculation by needle-pricks, and the entire 

 plant was destroyed in 6 days (see vol. I, plate 26). These plants were in pots in the hot- 

 house, were 1 to 1.5 feet high, and were growing rapidly when the stems were pricked. 



Fig. 18.' 



*Fig. 18. Angular leaf-spot on Rivers cotton. Inoculated by spraying Jan. 21-23, '95- Photographed March 

 15. Natural size. Spots in water-soaked stage. 



