1 920.] The White Rot Disease of Onion Bulbs. 1095 



soil, the attack commences with individual plants located 

 indiscriminately over the beds, but if the soil is heavily con- 

 taminated practically all the plants will be affected. Autumn- 

 sown plants appear to be the earliest to suffer, but spring-sown 

 onions suffer equally and, being smaller, are more quickly 

 killed and are sometimes destroyed in large numbers. 



The earliest visible symptom is one of wilting and yellowing 

 of the foliage. The oldest leaves turn yellow and fall over, 

 and later the other leaves also collapse. If the diseased plants 

 are gently pulled they will be found to come up very readily, 

 and closer examination will show that the roots have been 

 invaded by fungus mycelium or spawn, and have been almost 

 entirely destroyed (Figs, i and 2). It is probable that in all 

 cases the roots are attacked before the bulb. Warm, damp 

 weather appears to be specially favourable to the growth of 

 the parasite, and when these conditions exist the fungus 

 develops rapidly round the base of the bulb and soon covers 

 it with a mass of fluffy white fungus- spawn or mycelium 

 (Fig. I (c) ) . This white mycelium is very characteristic of 

 the White Rot disease, and distinguishes it at once from all 

 other diseases of the onion. When the disease is still 

 further advanced it will be found that the fluffy mycelium has 

 largely disappeared or has been replaced by a more closely 

 fitting weft which penetrates also into the tissues of the bulb 

 (Fig. I (a)). At this stage the surface of the bulb shows the 

 presence of numerous, black, spherical bodies about the size of 

 small poppy seed (0.5 mm. diam.) and termed sclerotia. The 

 sclerotia are the resting bodies of the fungus (corresponding 

 in a rough way with seeds), and it is by means of these that the 

 parasite persists in the soil from year to year. The fungal 

 threads continue to penetrate the tissues of the bulb, devouring 

 the cell-contents, and forming numerous sclerotia, until finally 

 the bulb becomes shrunken and dried up. The sclerotia are 

 produced in large numbers on the diseased plants, and it is 

 obvious that if the bulbs are left in the ground the soil becomes 

 thoroughly contaminated with the fungus. 



White Rot is most in evidence from the beginning of June to 

 early August. By August it has mostly killed or destroyed the 

 affected plants, and comparatively few new infections appear 

 to take place after that date. 



Method of Rtproduotlon and Over-wint«rlnsr. — In order to combat 

 the disease intelhgently it is essential to know the life- 

 history of the parasite and its methods of propagation and 

 perpetuation. 



