Anguillulina dipsaci on onion seed and its control by fumigation with methyl bromide

Abstract

Further investigations on the occurrence of Anguillulina dipsaci on onion seed are presented. It is shown by the examination of 2,000 seeds individually that the parasite is usually located on the seed in the region of the hilum. A given seed may have as many as 4·7 worms or only 1 worm attached to it.A method of seed fumigation with methyl bromide is described whereby the parasites attached to seeds are killed but the seed is unharmed. By the use of a test-bottle technique it has been found that a minimum dosage of 600, i.e. a concentration-time product of 600 involving an exposure to the fumigant of 18 to 24 hours at 24°C. is necessary for the destruction of the parasites attached to the seeds and in such flower-part débris as may be mixed with it.The fumigant has no harmful effect on the vitality of the seed at dosages sufficient to destroy the parasite nor at much higher dosages. There is thus a wide upper margin of safety in its use. Two practical methods for the fumigation of large quantities of seed are described.The probable origin of seed-borne infection from lightly infected parent bulbs is discussed. It is shown that at the present time seedborne infection is fairly widespread and has been found to occur on at least 9 different varieties of onion seed as distributed by seed merchants. Recommendations as to control are made which it is suggested would go far to reduce the incidence of the disease in this country.