Blister Rust on White Pine 



455 



burns and severe single burns destroy 

 the seed and crowns, and create ribes- 

 free conditions except in wet and rocky 

 places where they may survive the fire. 

 The production and longevity of ribes 

 seed, disturbances of the forest floor, 

 shade, plant competition, and fire are 

 factors in ribes suppression that receive 

 careful consideration in planning con- 

 trol operations and forest-management 

 practices. 



SPORES, the reproductive bodies of 

 fungi, serve the same purpose for fungi 

 that seeds do for ordinary crop plants. 

 The blister rust spores are minute, 

 dustlike particles that are easily carried 

 by the wind. One kind of spore, pro- 

 duced in the diseased bark of white 

 pines in the spring, cannot infect pines; 

 they infect only ribes. 



On ribes leaves, two kinds of spores 

 are produced, an early- and a late- 

 summer form. The early form can in- 

 fect ribes leaves but not white pines. 

 It is a repeating form, producing sev- 

 eral generations in a season and caus- 

 ing local disease intensification on 

 ribes. The late form infects white pine 

 needles but not ribes. The fungus grows 

 in the needles until it reaches the bark. 

 There it causes spindle-shaped diseased 

 areas called cankers. From 2 to 5 years 

 after infection of the needles, spores 

 begin to develop in the diseased bark. 

 Thereafter each spring a new crop of 

 spores is produced; they infect ribes 

 and again start the life cycle of the 

 fungus. 



Blister rust may reach new localities 

 by shipment and planting of infected 

 white pine or ribes and by wind-borne 

 spores from infected pines. Distance is 

 not a limiting factor in the spread of 

 the disease by shipment of infected host 

 plants. Investigations in western North 

 America showed that the disease was 

 spread by wind-borne spores from in- 

 fected pines to ribes over distances of 

 several hundred miles. From infected 

 ribes to the pines, however, the spread 

 usually does not exceed 900 feet. 

 Under especially favorable weather 

 and topographic conditions, the spread 



sometimes extends for a mile or more. 

 The amount of pine infection rapidly 

 lessens as the distance from diseased 

 ribes increases. 



The different native white pines are 

 highly susceptible, although there is 

 some evidence of resistant trees among 

 species. Native ribes species vary widely 

 in their reaction to the rust. Some are 

 highly susceptible, while others seldom 

 take the disease. That fact, however, 

 has had no significant effect on the 

 spread or control of the disease, be- 

 cause susceptible ribes species are well 

 distributed throughout the range of the 

 white pines. 



In new localities the disease follows 

 a definite course. It starts as a single 

 infection or several scattered infections 

 on ribes and is transmitted to nearby 

 white pine. Two to five years later the 

 diseased pine produces spores that in- 

 fect nearby ribes. In turn, the local pine 

 infection increases. After this situation 

 has developed in several spots, a favor- 

 able rust year causes abundant and 

 widespread infection on the ribes and 

 pines, and many pines die. 



The white pines would be doomed 

 within a few years were it not that the 

 rate of spread of the fungus depends on 

 the simultaneous occurrence of several 

 factors : Widespread infection on ribes, 

 abundant production of pine-infecting 

 spores, and a favorable combination 

 of temperature and moisture condi- 

 tions. The integration of such condi- 

 tions over extensive areas fortunately 

 are infrequent, and new infection of 

 pine is light in most years. Only in an 

 occasional year is it so general as to 

 cause widespread damage. 



THE CONTROL OF BLISTER RUST and 

 similar epidemic diseases, we believe, 

 is largely a public problem because of 

 their effect on national welfare, their 

 interstate distribution, and the need for 

 coordinated effort in their control. 

 The forest resources attacked by blister 

 rust are spread over many States and 

 involve lands in Federal, State, and 

 private ownership. Ribes must be re- 

 moved over extensive areas regardless 



