FACTORS AFFECTING THE INCIDENCE AND SPECIATION OF FUNGI COLONIZING WALNUTFRUITS Sherman

We investigated several cultural and environmental factors during 1977 which were thought to influence the incidence of moldy walnuts. These studies were meant to be a preliminary survey to determine potential areas of research to pursue. Results of these investigations are not to be considered as control recomnlendations since the studies represent only one year's data and especially during a year when no significant rain occurred during the harvest period. The areas of investigation included: Injury Nuts were injured during development with deep and shallow wounds and harvested at the normal time. We also sampled nuts with sunburn or blight damage. Nuts with deep hull injuries and sunburned and blighted nuts had a significantly higher % of mold. Alternately, the shallow wounds or wounds which healed over did not increase mold. These results suggest that large wounds or necrotic tissues provide an infection court or substrate base for invading fungi or a breakdown of the natural host defense mechanism. Early season injuries did not significantly increase mold over the late season injuries. Time of harvest Nuts were sampled from Ashley, Serr and Tehama varieties at one location, 2 and 3 weeks prior to harvest, at harvest time, and 2 and 3 weeks after harvest. There was no significant difference in the % moldy nuts at any of the harvest dates. The Tehama variety showed a slight but not significant increase from 0 to 14% mold. Ethephon Previous studies indicate that nuts harvested early using ethephon have significantly less mold. Since ethylene gas released from ethephon is potentially fungistatic we wanted to determine if the reduced mold was due to the early harvest only or also due to a fungistatic effect. There was no significant difference in the % mold in nuts harvested 4 or 7 days earlier than the normal harvest either treated or non-treated with ethephon. Preliminary lab studies have shown no obvious effects of ethylene gas on the growth of fungi isolated from walnuts. Nuts on the ground Unhulled nuts placed on the ground for 2 weeks had 59% mold compared to 10% in nuts sampled directly from the tree at the time nuts were placed on the ground. Hulled nuts left on the ground for 2 weeks were not always significantly higher in % mold. Nuts placed on the ground for 1 week (hulled or unhulled) did not have more mold than the controls. Storage of nuts in bins Freshly harvested, hulled nuts were stored in a bin for 12 days without a significant increase in the % mold. A companion bin was wetted daily for the 12 day interval. During the first 4 days there were no major differences between the % mold in the wet or dry bins. However, on the 5th day the nuts sampled from the wetted bin showed a dramatic increase from 8 to 68% moldy nuts. The % moldy nuts from the wetted bin was usually over 50% throughout the 12 day period and was 91% at the termination of the study compared to only 1% in the dry bin. Isolation of fungi from developing nuts Nuts sampled from July 11 to August 31 showed 4% (3/79) of the kernels intectea with fungi and 87% (69/79) of the hulls infected. Fungi were of several species and 80% grew from the stem or blossom end of the hull. However, by October 4, even though hulls were not yet split, 97% (30/31) of the kernels were infected with fungi and 92% (11/12) of the kernels sampled October 18 with the hulls split were infected. Generally there was no macroscopic evidence of fungi prior to incubating the nuts.