ELIMINATION AND SEPARATION OF VIRUSES IN APPLE CLONES BY EXPOSURE TO DRY HEAT

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Abstract
Virus infections have been eliminated from apple clones by propagation of lateral buds from potted apple trees after their exposure to dry heat at 38 °C for varying periods. For several clones the minimum heat exposure period imposed (7 days, following preheating 7 days at 35 °C) inactivated viruses that cause stem pitting of Virginia crab; foliage and shoot symptoms on Hopa crab; rubbery wood on Lord Lambourne apple; foliage and dieback symptoms on Spartan apple (McIntosh pucker virus); and leaf pattern, epinasty, and bark necrosis of Spy 227.The apple clones displayed wide differences in heat tolerance. Minimum heat exposure periods for virus inactivation varied among clones treated. For two clones, the buds sampled after maximum heat exposure periods, approaching survival threshold of the plants, gave higher levels of positive indexing on Spy 227, Hopa crab, and Virginia crab than those sampled after short or intermediate periods. Virginia crab stem pitting and Virginia crab decline were shown to have distinct etiology: the virus causing stem pitting proved very sensitive to heat, whereas that causing decline was heat-tolerant. Indexing before and after heat exposure provided evidence that distinct viruses or strains induce the reactions of Hopa crab and Spy 227, Hopa crab and Virginia crab, and probably also Virginia crab and Spy 227.Indexing of internodal bark chips at the time buds were removed from the heat chamber for propagation yielded readings quickly and provided a generally accurate forecast of the indexing results for trees derived from the treated buds.

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