Dec. 9, i9i8 Seedling Diseases of Conifers 525 



Both these types of damping-off of tops commonly kill the affected 

 seedlings, but not always. Stem infections regularly cause death, but 

 lesions originating in the cotyledons are often arrested before they 

 progress far enough to kill the growing point in the center of the whorl 

 of cotyledons. 



BLACKTOP 



This type of damping-off known as black top is simply a special case 

 of the preceding from which it is distinguished by the dark color of 

 the tissues decayed. It is illustrated in Plate B, figures 2 and 3. Infec- 

 tion takes place at any point on the stem or cotyledons of very young 

 seedlings. Both infection and the extension of the lesion seem to 

 depend on special weather conditions. There is reason to believe that 

 a species of Trichoderma is the direct cause. This type of damping-oft' 

 is seldom sufficiently prevalent to be of importance. Soil disinfection 

 with acid is apparently ineffective in preventing blacktop damping-off. 



DECAY OF DORMANT SEED 



It is undoubtedly true that dormant seed are sometimes killed by micro- 

 organisms. Under ordinary seed-bed conditions, species of Pythium 

 and Corticium probably kill some coniferous seed before the coats are 

 split, although this has yet to be demonstrated. It is also considered 

 probable that under certain conditions considerable quantities of seed 

 are destroyed by ordinarily saprophytic molds, which possibly reverse 

 the usual action of the damping-off parasites by attacking the cotyledons 

 before the embryonic radicle is invaded. Seed of jack pine (Pinus 

 hanksiana) shipped moist in a tin container has been found seriously 

 molded with a species of Penicillium fruiting vigorously on the outside 

 of all the affected seeds. It is commonly stated that the seeds of many 

 crop plants rot in the soil during prolonged wet weather. To what 

 extent coniferous seed suffer from microorganisms before beginning to 

 germinate can not be stated without further investigation. Although 

 perhaps not strictly a damping-off problem, it is certainly one that 

 can be considered to advantage in connection with work on damping- 

 off proper. 



TYPES OF DAMPING-OFF' IN FORESTS 



In direct seeding by the seed-spot method in northern Idaho, the 

 writers are advised by Mr. E. C. Rogers, of the Forest Service, that 

 a considerable proportion of the seedlings die from what appears to be 

 damping-off, and that cultural examinations have shown Rhizoctonia 

 sp. in the majority of the cases. 



In natural reproduction normal damping-off at least in most forests 

 appears to be a much less important factor than in nurseries. Serious 

 damping-off should be expected only where seedlings come up in groups, 

 or where soil corditions especially favor parasites. 



