BLIGHTS OF CONIFEROUS NURSERY STOCK. 6 



occurred at nurseries in Colorado, Minnesota, Pennsylvania, and 

 New York without being recognized by the nurserymen. 



HOW SUN SCORCH WORKS. 



In serious cases of sun scorch, seedlings of all ages are killed out- 

 right. In less serious cases part of the plant is killed. Either the tips 

 or the lower needles may be first affected. If the tips are injured, 

 stem and bud may be killed as well as the needles, so that growth 

 can be resumed only by lateral buds below the point of injury. In 

 slight attacks only the needles, and often only the tips of the needles, 

 are killed. When the lower needles are the only ones attacked, 

 despite the fact that they are well protected from sun and wind in 

 dense stands, it must be assumed that the younger needles at the 

 tips, subjected to much more light and Wind, have survived at the 

 expense of the lower ones by taking more than their share of the 

 available water. Affected needles first turn slightly yellowish, lose 

 their green until they have become a pale straw color, and then grad- 

 ually turn deeper brown until they become nearly red. In summer 

 most dead needles fall within a month. 



In crowded seed beds, especially on very sandy soil, attacks may 

 take place very suddenly in hot, dry weather, and patches of seed- 

 lings up to a foot in diameter may be entirely killed. It is these 

 .definite, clean-killed patches which have most often caused the disease 

 to be called parasitic. Such even death can be explained on a drought 

 basis only by assuming that in such cases the root systems of all the 

 seedlings involved have been equally extended in normal competition 

 for water, just as the tops often grow to an exactly even height in 

 competition for light, so that all the seedlings in the patch are on 

 terms of equality when the competition becomes critical. When part 

 of the seedlings in a crowded bed are badly suppressed the smaller 

 seedlings are in some cases, when transpiration is rapid, even less 

 subject to sun scorch than the larger. Since the suppressed seedlings 

 presumably have less extensive root systems, their endurance in such 

 cases must be explained by the fact that they are also less subject to 

 water loss because of the protection afforded by the tops of the larger 

 plants around them. When the beds are not crowded and the 

 weather does not favor extremely rapid water loss, death does not 

 occur so suddenly or in such definite patches. 



In the cases of transplants a certain number die soon after they are 

 set out and before root growth starts. This has been called " trans- 

 planting loss " and is not included under the term " sun scorch." 

 " Sun scorch " is used in transplants for death due to excessive water 

 loss after the plants have become partly established and commenced 

 growth. The work of sun scorch in transplant beds at Halsey differs 

 from that in seed beds in that the trouble in the transplants does not 



