68 



BACTERIA IN RELATION TO PLANT DISEASES. 



certain course, which is usually not shorter than several weeks, the disease stops and the 

 organisms which caused it are then found to be dead in the blighted tissues (pear-blight). 

 This, however, does not seem to protect the tree from new infections the following year, 

 i. <?., the disease is not self -limited and protective from new infections like the eruptive fevers. 

 Not infrequently rapidly growing, juicy trees of pear, apple, quince, loquat, and mulberry 

 are killed outright in the course of one season if left untreated. Even whole orchards have 

 been destroyed, as in Georgia and California. Olive-tubercle also sometimes kills young 

 trees, but more often it kills only some of the smaller branches and renders the tree unfruit- 

 ful. Certain infections seem to kill almost infallibly. This is true of Bacillus tracheiphilus 

 in musk-melons and cucumbers, and of virulent strains of Bad. solanacearum in young 



Fig. 20.* 



tomatoes, potatoes, and egg-plants. Whole fields of potatoes, tomatoes, and tobacco when 

 young may succumb quickly to this disease, particularly in moist soils containing root 

 nematodes. In carefully made inoculations on young plants, using either of these organisms, 

 at least 95 per cent of the infections are promptly fatal, i. e., within 2 or 3 weeks from the 

 first visible signs of the disease, and sometimes much sooner (see vol. I, plates 24 to 27). 

 Old plants are more resistant, especially to Bacterium solanacearum. In the same way old 

 and slow-growing cabbages are rather resistant to Bacterium campestre and may not be 

 wholly destroyed, but young and rapidly growing plants are very apt to die either from the 

 direct effects of the parasite or from the action of the soft-rots which follow it. 



*FiG. 20. — Coconut budrot of Eastern Cuba. Outer haf-sheaths removed to show inability of diseased terminal 

 bud to support its own weight. Tree No. lo. Photographed at Baraeoa, Cuba, April i8, 1904. Natural size. Photo- 

 graphed in a room at 2 p. m., raining, with Cramer's isoinstantaneous plate, Zeiss double protar lens, series viia, 

 stop 256, time 30 minutes. 



