Nov. is, 1920 Crownwart of Alfalfa Caused by Urophlyctis alfalfae 319 



CLIMATE IN RELATION TO THE DISEASE 



The fact that the disease has apparently remained so long limited in 

 its distribution to certain regions in the western portion of the country 

 without invading the larger alfalfa-growing areas in the central portion 

 of the country raises the question whether this limitation is due to cer- 

 tain climatic conditions which favor the development of the fungus in 

 these localities or to some factors which have prevented the spread of the 

 causal organism. That the spread of the organism has been inhibited by 

 lack of facilities for distribution is hard to imagine. Even if it should be 

 found that the spores are incapable of withstanding the drying incident 

 to being transported with seed or hay, still a considerable number of 

 plants have been and still are transported by individuals for trial or 

 experimental purposes, and it is hard to believe that no warted plants 

 have been sent at some time into the central and eastern States. On the 

 other hand, it is not easy to discover any common factors of climate in 

 the regions where the disease now occurs which do not exist in the larger 

 eastern regions. For the most part, the disease exists in valleys where 

 the winter is very mild and where there is at least a slight growth of the 

 plant during every month of the year. Such conditions would seem to 

 furnish a long period favorable for infection. However, the disease also 

 occurs in the Salt Lake Valley in Utah and in certain high mountain 

 valleys where the winter is severe. The mere fact of severe winter does 

 not seem to be the sole limiting factor. Thus, it is not possible to answer 

 with an opinion based upon suitable evidence the most important ques- 

 tion from an economic point of view that is being asked regarding the 

 disease. Of course it might be determined decisively whether the dis- 

 ease can develop in the central and eastern portions of the country by 

 bringing diseased plants into these regions and observing their behavior. 

 Fear that such experiments might result in a destructive spread of the 

 disease has prevented the initiation of such experiments thus far. 



CONTROL MEASURES 



Thus far no experimental work bearing directly upon control meas- 

 ures has been undertaken. The direction which such experimental work 

 should take appears to be clearly indicated by the observation of the 

 field conditions under which the disease now becomes most abundant. 

 The one condition which more than any other appears to favor the 

 development of the disease is an excess of moisture in the soil in the 

 early spring when it appears that infection must take place if at all. 

 Any measure which will avert this excess, as by drainage or a dimin- 

 ished supply of irrigation water, should bring about a reduction in the 

 amount of disease. 



Under some conditions deep cultivation may reduce the disease. In 

 the spring of 191 8 some fields which had received a thorough and deep 

 9508 — 20 6 



