Graduate Thesis Or Dissertation
 

The use of flowering crab apples for the detection of latent viruses in apples

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  • Recent investigations indicate that several latent viruses of apples exist in common commercial apple stocks, but these viruses and their relationships are incompletely understood. This thesis describes investigations of crab apples as indicators of latent viruses of apples. Eighty-eight varieties and species of crab apples were tested as indicators of latent virus of apples. Seventeen reacted to a single source of inoculum white 71 remained symptomless. Of those that reacted, five gave a severe reaction, six gave a moderate reaction, and six gave a mild reaction to this inoculum. Twelve of the 17 reactors were selected and inoculated with five selected inocula. These inocula produced a similar range of symptoms on the 12 indicator varieties. These results suggested that the crab apples were sensitive to the same virus or viruses. The crab apples that reacted severely to the virus were often killed before sprouting and therefore the results were inconclusive. Delaying inoculation for 14 days after planting in the greenhouse allowed the plants to start growing and produce a more conclusive virus reaction. The species M. huphensis was the most sensitive indicator in these trials. This indicator was capable of detecting a virus present in normal-appearing Hopa crab appIe, another indicator of latent virus of commercial apples. The indicator M. huphensis is recommended as a preliminary screening host in the search for disease-free varieties. Using single virus isolates of Chlorotic Leaf Spot, Spy Lethal, Stem Pit, and Line Pattern viruses, evidence is presented to show that the virus causing the reaction on crab apples is not identical with any of the viruses named above. The Bartlett pear seedling 138, an indicator for Pear Mosaic virus, is recommended as a filtering plant for the virus causing the reaction on Hopa crab apple. This virus can be removed frorn a culture by inoculating donor buds of the culture onto the Bartlett pear seedling 138. Data are presented indicating that Pear Mosaic virus is not identical with Chlorotic Leaf Spot virus nor is it identical with the virus causing the reaction on Hopa crab, Chlorotic Leaf Spot virus cannot be suitably detected under the conditions described in these experiments. These tests were designed for a maximum length of 60 days, and Chlorotic Leaf Spot virus sometimes fails to cause symptoms on Russian seedling R12740-7A until two years after inoculation. Movement of a latent virus of apple was followed in a newly infected apple seedling. The rate of movement was rapid with most of the seedling invaded within seven weeks after inoculation in most cases. The pattern of movement within the seedling was not uniform. Tissues adjacent to the site of inoculation were often free of the virus while more distant tissues were readily invaded. Knowledge of latent viruses of apple is still confused, and extensive work is needed with many indicators to determine if many strains of a few viruses or many individual viruses are involved. The list of indicators for latent viruses of apple is not complete, but the crab apples are an important part of this list.
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