NURSERY PRACTICE FOR PRAIRIE-PLAINS PLANTING 147 



nation and may be helpful for some species in reducing damping-off 



losses. 



It has been found that certain seed dusts are fairly effective in 

 reducing preemergence damping-off. The most satisfactory dust so 

 far used has been zinc oxide, applied at the rate of 2 ounces per pound 

 of air-dried seed (not green or wet seed). The kind of zinc oxide is 

 very important since not all sources have proved equally satisfactory. 

 Further advice should be secured from plant pathologists before 

 attempting extensive treatments. Copper oxide is also fairly effective 

 in controlling preemergence damping-off losses, but broadleaf species 

 have been found to be susceptible to injury by this dust. American 

 elm is especially susceptible to injury, while Siberian elm is somewhat 

 less detrimentally affected, perhaps because of its more rapid germi- 

 nation. Copper oxide appears to retard germination of desertwillow 

 seed only slightly and has been found effective in controlling pre- 

 emergence losses for this species in greenhouse tests. For desert- 

 willow, however, commercial dry bordeaux mixture, applied at the 

 rate of 2 ounces per pound of dry seed, is probably safer and is just 

 as effective in controlling preemergence damping-off as copper oxide. 

 Field trials in Texas have substantiated the greenhouse results. 

 Formaldehyde seed dusts applied at time of sowing have proved 

 unsatisfactory both in the greenhouse and in field tests. None of the 

 seed dust treatments can be considered entirely satisfactory, since 

 they fail to retard post emergence damping-off losses. Mixing the 

 dusts directly with the soil is a more satisfactory treatment but is 

 costly for large areas. 



Formaldehyde soil treatments are quite effective on sandy soils in 

 controlling both preemergence and post emergence damping-off losses 

 but in general are too costly and cumbersome for the extensive opera- 

 tions of the present Federal deciduous seedling nurseries. For seed- 

 lings grown in small beds, commercial formaldehyde used at the 

 strength of 1 part in 50 parts of water, applied at the rate of 1 pint 

 per square foot 10 days prior to sowing, has given satisfactory results. 



Taking all factors into consideration, such as cost, ease of appli- 

 cation, and final results, it appears at this time that the most practical 

 method of controlling preemergence damping-off losses of deciduous 

 tree seedlings is by the use of zinc oxide as a seed dust. Desert- 

 willow is an exception, and commercial bordeaux dust mixture is 

 recommended for this species. No satisfactory method for con- 

 trolling postemergence losses over large areas has yet been developed. 



Root Rot 



Some root rots (sometimes referred to as "late-damping-off") are 

 difficult if not impossible to avoid, even by the most careful selection 

 of nursery sites. The so-called late root-rotting fungi are referred to 

 here. They are unusually widespread and do not necessarily infect 

 indicator plants. Seedlings up to several months old are still subject 

 to late root rot. In such eases the plants do not damp-off since the 

 stems are now strengthened by the formation of woody tissues. 

 Typical symptoms are indicated by a drying and curling-up of the 

 leaves, usually followed eventually by a wilting of the tops and the 

 death of the seedlings, although sometimes the plants survive but 

 are badly stunted. 



