18 MISC. PUBLICATION 4 4 0, U. S. DEPARTMENT OF AGRICULTURE 



where it attacks garlic; and elsewhere throughout Europe. Until 

 1923 it had been reported from only two onion districts in the United 

 States, having been found in Oregon in 1918 and in Virginia in 

 1923. Since that time it has caused more or less trouble in Cali- 

 fornia, 4 Virginia, 5 Kentucky, and New Jersey. The disease is of 

 importance in the growing crop as well as after harvest. 



The first symptoms of the disease in the growing plant are yellow- 

 ing and dying back of the leaves, the rate of progress depending 

 upon environmental factors. The affected plant, especially if young, 

 may wilt and collapse completely, or there may be a gradual decline 

 which extends through several days or weeks. If a diseased plant 

 is pulled it will be found that the roots and scale bases are decaying. 

 The affected roots generally die and the scales become semiwatery. 

 The symptoms displayed by the bulb often are like those of other 

 bulb rots, such as gray mold rot and fusarium bulb rot. However, 

 the mycelium is more white and cottony and the sclerotia are much 

 smaller (one thirty-second of an inch in diameter) and more spherical 

 than those of the Botrytis species that cause bulb rot (pi. 5, F). In 

 fusarium bulb rot no black sclerotia are produced. 



CAUSAL FACTORS 



The pathogen, Sderotium cepworum, lives over winter in the soil, 

 from which it attacks the new crop. It can also live over in bulbs 

 and thus be carried into noninfested fields. It is readily carried 

 from field to field by infected bulbs, infected seedlings, and by con- 

 taminated containers. 



Field observations and laboratory experiments indicate that the 

 disease is most destructive in a moderately cool soil (50° to 68° F.) 

 having a medium moisture content. Such conditions prevail in 

 the onion sections of the Southern States. Whether the climatic 

 conditions in the North are such as to prevent establishment of 

 the disease remains to be discovered. Temperatures below 41° 

 and above 84° prevent the growth of the causal organism. 



No data are yet available regarding the development and spread 

 of this disease in transit and storage. 



CONTROL MEASURES 



The sclerotia of the white rot fungus can remain alive in the soil 

 for several years; therefore fields known to be infested should be 

 avoided for planting onions. 



Shallots and leeks are said to be more resistant than onions. 



(See 12, 47.) 



PULSE FAMILY 



The pulse family (Leguminosae) contains a large number of 

 cultivated plants many of which are important vegetable and forage 

 crops. In this group there are several genera and species of beans 

 and peas that produce fruit pods and seeds which are classed as 



* Gardner, M. W. sclerotium cepivorum found on garlic in California. U. S. Bur. 

 Plant Indus., Plant Dis. Rptr. 23:36. 1039. [Mimeographed.] 



r. S. Bureau of Plant Industry, white rot of onions in Virginia. U. S. Bur. 

 Plant. Indus., Plant Dis. Rptr. 14: 104-105. 1030. [Mimeographed.] 



