BUREAU OF PLANT INDUSTRY. 185 



cooperatively witli imich greater effectiveness than in any other 

 way. By arranging cooperative activities in this phase oi" the potato 

 work with the experiment stations that are located in tlie iinpoitant 

 potato-producing States a local interest is created, antl many more 

 growers are able to see for themselves the results of i)roper selection 

 of seed stock than would be possible if this work were done in a single 

 location. 



Mosaic aiul leaf -roll of the potato. — Investigations of the so-called 

 degeneration diseases — mosaic, leaf-roll, and related troubles of ])o- 

 tatoes — which have been in progress for several years, show that 

 this group of maladies has become the greatest handicap to potato 

 improvement and causes great loss to producers throughout the 

 country. The outstanding feature of the results secured is the dis- 

 covery that apliids are the most effectiA'e natural means in the trans- 

 mission of these diseases. In addition, it has been found by an in- 

 vestigator at the Maine Agricultural Experiment Station that the 

 principal overwintering host of the potato aphis is the rose. Last 

 season's observations indicated that both streak and curly-dwarf are 

 closely related to mosaic and similar diseases of the potato, being 

 frequently associated with them in the field and capable of trans- 

 mission from diseased to healthy plants by juice transfer. Observa- 

 tions have also shown that the symptoms of mosaic vary consider- 

 ably on different varieties under different climatic conditions and 

 at different periods of the season. Plants becoming infected during 

 the latter part of the growing season, when growth has practi- 

 cally ceased, will not exhibit any of the symptoms of mosaic, 

 but the tubers from such hills will produce infected plants the 

 following season, the disease being more severe it^ the affected stock 

 is planted in the South. Last season's experiments on the control of 

 mosaic by roguing confirm previous results, showing that roguing 

 under ordinary field conditions, with mosaic plants in adjoining 

 plats, will not free such stock from the disease but will tend to 

 reduce the percentage of diseased plants, provided it is done thor- 

 oughly and continued throughout the season. Roguing stock in lo- 

 calities Avhere the percentage of mosaic and aphid infestation is low 

 has resulted in reducing the mosaic from 10 per cent to 1 or 2 per 

 cent in one season. Different varieties react differently to the dis- 

 ease, but there is less resistance than was formerly thought, and 

 it is necessar}^ to know the foliage reaction of each kind. 



NEW VEGETABLES. 



Work on the introduction and establishment of the dasheen, 

 chayote, and tropical yams has been carried forward vigorously, 

 and many experimenters have been supplied with material for prop- 

 agation. The market for dasheens is growing, not only among our 

 foreign-born population who have been familiar with these vege- 

 tables in their early homes but also among native Americans. Com- 

 mercial shipments totaled about 10 carloads last season, A strong 

 demand for tropical yams has developed since their importation was 

 prohibited in 1918, and the problem now is to secure the best varie- 

 ties for cultivation in this country and to find farmers who will 

 grow this crop on a cornmercial scale. Cultivation is limited to 

 Florida and adjacent States. When properly prepared, the greater 



