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. oleraceae, 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. destructans 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 Fig. 18.* 
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. 
*Fic. 18.—Angular leaf-spot on Rivers cotton. Inoculated by spraying Jan. 21-23, 1905. Photographed March 
15. Natural size. Spots in water-soaked stage. 
