1893.] PUBLIC DOCUMENT — No. 33. 243 



the result is that it is girdled, and the whole of it beyond 

 the point of girdling dies. The attacks of this fungus on 

 its host-plant are essentially similar in their results to those 

 of the black knot on the plum, though the immediate effect 

 on the inner bark is here one of atrophy, while in the latter 

 case it is one of hypertrophy. The present fungus is also 

 nearly related to the black-knot fungus, but its life history 

 is not yet at all known. What other spore forms constitute 

 stages in its life cycle we have yet to learn. Therefore, it is 

 impossible to give any more definite suggestions for avoid- 

 ing it than to recommend that infected branches be cut 

 away well below the point of infection and burned as soon 

 as they are seen to be infected. This precaution, if taken 

 in season, will prevent the dissemination of the spores 

 described above, and should thus prevent the development 

 of other spore forms dependent on them, and the infection 

 of new branches, 



VII. Treatment for Powdery Mildews. 



In pursuance of the plan indicated in our last report,* we 

 conclude the present one with recommendations for the 

 avoidance of loss from the attacks of some chosen groups 

 of fungi. The group selected in the present case is that 

 embracing those fungi which are known as the powdery mil- 

 dews. These parasites attack a considerable number of 

 cultivated plants, and, as has been seen, sometimes cause 

 extensive losses. They are among the commonest fungi of 

 the greenhouse, but are equally common in summer, in the 

 open air. Among the plants most often attacked by some 

 one of the powdery mildews in the greenhouse are the 

 cucumber, grape, rose, verbena and other florist's plants. 

 Out of doors, the pea, gooseberry, strawberry, and young 

 plants of the apple and cherry often suffer from them. 



The appearances produced on their host-plants by three 

 species of powdery mildews have already been described in 

 tills report. In general, it may be said that it is in the 

 summer spore stage that these fungi are most harmful as 

 disease producers. In this stage they may form dense white 

 "floury" patches on the host, usually on its leaves, or the 



* Ninth Report Massachusetts Experiment Station, pp. 243, 244. 



