1920.] The White Rot Disease of Onion Bulbs. 



1097 



to make an exact statement as to this, since infected soil has 

 not been long enough under examination. From accounts 

 collected from market gardeners and others it would appear 

 that the fungus persists in the soil for a considerable period, 

 at least three or four years, and, judging by analogy with 

 allied species, the sclerotia may survive considerably longer. 



Method of Spread. — The introduction of the disease into new 

 localities obviously takes place through such means as the 

 distribution of soil containing sclerotia, the planting of diseased 

 plants and sets, or through the careless disposal of contaminated 

 refuse or manure. Local spread is probably chiefly accounted 

 for through the use of contaminated manure and rubbish, 

 or the scattering of affected soil. For the spreading over larger 

 areas the distribution of diseased seedlings is no doubt mainly 

 responsible. Seedling onions for planting are brought to market 

 in quantities in April and May. These are often reared on in- 

 ■ fected land, and on more than one occasion the white mycelium 

 of the White Rot has been noted on retail supplies. The 

 diseased plants are distributed to gardens and allotments ; 

 and, though seedhng plants probably produce but relatively 

 few sclerotia, a sufficient number are developed to contaminate 

 the soil, so that with subsequent crops of onions the tendency 

 is for the disease to increase year by year. This is especialty 

 the case when onions are repeatedly grown in the same land, 

 and very much less so when a long interval is allowed. The 

 disease may also be spread by the use of slightly diseased 

 " sets," when the attack is so slight as not materially to injure 

 the bulb or to attract attention. 



Susceptibility of Varieties and otiisr Crops. — As far as the 



present observations go, most of the common varieties of 

 onions appear to be subject to White Rot. Shallots, as a rule, 

 are markedly resistant, and have been observed growing side 

 by side with diseased onions, but remaining perfectly free from 

 attack. On one occasion, however, shallots grown at Kew 

 were affected and a number of bulbs were completely killed, 

 and one other case of this crop being diseased has been observed. 

 Leeks also do not appear to suffer as a rule, but two instances 

 of shght attacks on leeks have been noted. 



Distinctions between Wliite Rot and other Diseases. — The effect 

 of \\Tiite Rot as seen in the foliage somewhat resembles an 

 attack by the maggot of the Onion Fly. That disease, however, 

 is distinguished by the presence of the n aggots in the bulbs, 

 and the absence of the white fungus myceUum and the black 



