ONION SCAB 273 



ONION SCAB 



( Vermicularia circinans. Berk.) 



This is an erratic parasite, being very abundant during 

 certain seasons, and entirely absent at others. As a rule 

 but little injury is done to the bulbs, which are the only 

 part attacked, so long as they remain in the ground ; never- 

 theless the unsightly black blotches on the outermost 

 scales considerably reduce their market value. 



The fungus usually appears when the bulbs are nearly 

 full-grown, under the form of scattered black patches, 

 formed of minute black, velvety tufts, arranged in con- 

 centric circles, or irregular, wavy lines. If one of these 

 mature velvety tufts is examined under the microscope, it 

 is found to consist of myriads of erect threads, each bear- 

 ing a long, slender, slightly curved, colourless spore at its 

 tip, and in addition the tuft is thickly studded with long, 

 black, projecting spines, which give to it the velvety 

 appearance when seen through a pocket-lens. 



Preventive Means. — Bulbs showing the least signs of 

 the disease should not be mixed with healthy ones, and the 

 bulbs should be thoroughly dry before storing, otherwise 

 the disease will spread quickly ; as, notwithstanding great 

 care, a few very slightly tainted bulbs are liable to be 

 passed ; and if there is the least indication of sweating, the 

 resulting moisture and heat greatly facilitate the growth 

 of the fungus, which often spreads with startling rapidity. 

 Diseased bulbs should not be allowed to decay on the 

 ground, but should be carefully collected and burned, 

 otherwise the spores remain in the soil for some time in a 

 condition ready for germinating ; and it must be borne in 

 mind it is only necessary that a single onion should be 



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