logS The White Rot Disease of Onion Bulbs. [feb.. 



sclerotia. The decay, moreover, is usually of the nature of a 

 wet rather than a dry rot (see Leaflet No. 31.) 



Of fungus diseases the most generally distributed is Mildew. 

 This occurs as a whitish-lilac or grey mildew on the leaves, 

 the bulb remaining free from attack (see Leaflet No. 178). 

 Onion Smut, a recently introduced and serious disease, is 

 distinguished by the presence of streaks of black spore-masses 

 in the- leaves and outer scales.* 



The disease with which White Rot is most Hkely to be con- 

 fused, except by expert mycologists, is that caused by the 

 fungus named by the American botanist, M. T. Munn, Botrytis 

 aim. In its typical form, however, the Botrytis disease is 

 easy to distinguish by the fact that (i) it is the leaves and the 

 top of the bulb rather than the roots and base of bulbs that 

 suffer ; (2) the mould which is present is grey and not white, 

 and (3) the disease usually commences later in the year, namely, 

 in late summer and autumn. Botrytis is particularly prevalent 

 in wet seasons, and on poorly ripened bulbs. It is, indeed, 

 often responsible for very serious losses in storage. Botrytis 

 agrees with the White Rot fungus in producing sclerotia, but 

 these are usually larger and flatter than in White Rot. 

 Occasionally the two diseases are found in the same bulb. 

 A full account of the Botrytis disease as it occurs in America 

 has recently been pubHshed by M. T. Munn (New York Agric. 

 Expt. Station (Geneva), Bull. 437, 1917). 



Distribution of the Disease. — In England White Rot is very 

 widespread, and is found especially in gardens, market gardens 

 and allotments. In the Report of the Ministry of Agriculture's 

 Plant Disease Survey for 191 8 it is listed from 15 counties; 

 it doubtless occurs in all parts of England. Mr. G. P. Berry, 

 of the Ministry of Agriculture, has observed the disease near 

 Edinburgh, and Dr. G. H. Pethybridge, of the Irish Depart- 

 ment of Agriculture, states in a letter that it is plentiful around 

 Dublin, but no published record exists of its occurrence in 

 either of these countries. In some of the market gardens near 

 London it is prevalent over extensive areas and causes much 

 trouble, especially amongst spring onions. Near Manchester 

 also it is said to cause very serious losses. From Voglino's 

 account the disease is evidently serious in Italy, but owing to 

 the confusion existing between White Rot and the Botrytis 

 disease Httle is known as to its exact distribution on the 

 Continent. No record appears to exist of its occurrence in 

 America. 



* See this Journal, May, I9i9» PP- 168-174. 



