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Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)

Abstract

Yellow dwarf of onions is a transmissible virus disease, of the mosaic group. The virus overwinters in the infected onion bulbs and in volunteer onion plants that live over winter in the fields. It probably is neither seed nor soil borne.

Yellow dwarf symptoms are characterized by yellowing and crinkling of the leaves, which become more or less flat and droop over in the advanced stages. Flower stalks of infected mother on ion plants become yellow, twist, and curl and are shorter than normal plants. Masking of yellow dwarf symptoms occurs quite commonly in infected onion plants, and these plants are a source of inoculum for infecting healthy plants; bulbs from the plants masking symptoms when regrown produce infected plants that show disease symptoms.

Plants grown from infected onion sets and those that become infected early in the growing season produce under-developed bulbs of little commercial value. Onion plants having masked infection throughout their growth period produce apparently normal plants and yields. Infected mother onion plants produce normal seed, but the seed yield is about 30 percent less than from healthy plants.

Yellow Dwarf is transmissible by artificial inoculation and insect vectors. By artificial inoculations the incubation period is usually about 10 days. Onion bulbs hypodermically inoculated with juice extracted from the fleshy and dry scale leaves of infected onion bulbs failed to show infection. Inoculations made during retarded growth periods either fail to infect the plants or the symptoms are not manifested during the current growth period. Signs of the disease appear when the bulbs of such infected plants are regrown. The virus extracted from yellow dwarf infected onion leaves that mask the symptoms is infective.

Yellow dwarf virus was transmitted under controlled conditions by the bean aphid (Aphis rumiscis Linn.), corn leaf aphid (Aphis maidis Fitch.), and the apple grain aphid (Rhopalosiphum prunifoliae Fitch).

The yellow dwarf virus was inactivated when the viriferous juice was stored in vitro at 29° C. for a period of 112 hours.

When infected onion leaves were stored in the open, at 29° C. for 100 hours, juice extracted from the leaves was non-infective.

The infectivity of the virus was only slightly retarded when viriferous juice was heated 10 minutes at 70° C. At 75° C. for 10 minutes, 55 percent of the original virulence of the virus was lost. At 80° C. for 10 minutes, the virus was inactivated. With 10-minute exposures the critical thermal point for the yellow dwarf virus lies between 75 and 80° C.

Exposures as low as -14° C. for 6 hours failed to inactivate yellow dwarf virus.

Inoculations with dilutions of 1-10,240 and above failed to infect any of the onion plants.

Yellow dwarf virus inoculated into Chinese sacred lily (Narcissus tazetta L.) bulbs, Jonquil (Narcissus jonquilla L.) bulbs and shallots (Allium sativum L.) visibly infected 60, 30 and 90 percent of the plants, respectively.

Inoculations with other plant viruses, including mosaic of Chinese sacred lily (Narcissus tazetta L.) and Jonquil (Narcissus jonquilla L.) failed to infect onion plants.

Only one (Riverside Sweet Spanish) of thirty-six onion varieties tested seemed to be markedly resistant.

The combined effect of indexing all growing stocks of onion bulbs, producing the planting stock of bulbs in areas free from yellow dwarf and roguing out the infected volunteer onions in the fields reduced the percentage of yellow dwarf infection in the district from 40 percent in 1928 to a trace in 1933 and 1934.

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