Fruit samples of nectarines and plums were collected from a total of 21 orchards (including prunes) in 1996 and 19 (peach, nectarine, and plum) orchards in San Joaquin Valley and nine prune orchards in both the Sacramento and the San Joaquin Valleys in 1997. These samples were processed using the overnight freezing-incubation technique (ONFIT) developed in our laboratory. This technique reveals latent infections of brown rot fungi (Monilinia spp.) in stone fruits. The incidence of latent infections was determined after thawing and incubating the fruit at 68º F for five to seven days. Because of unfavorable conditions for latent infections in both 1996 and 1997, it was impossible to choose three nectarine or peach orchards one each with low, medium, or high potential of brown rot to perform objectives two and three of the project. To increase the chance for detecting brown rot at different levels we increased the number of orchards sampled from the proposed 10 to 21 (1996) or 28 (1997), including the prune orchards. But still latent infections in most of these peach, nectarine, and plum orchards were extremely low. However, latent infections were higher in prune orchards (particularly in 1996) and we were able to complete Objectives 1, 2 and 3 in both 1996 and 1997. Each year we used one French prune orchard each with expected low, medium, or high potential for brown rot. |
In the majority of the samples, latent infections detected by using the ONFIT were zero, only on a few occasions, the incidence of latent infections in samples from peach and nectarine orchards reached 5% (1996) or 3% (1997). However, in 1996 the incidence of latent infections in French prune was high in the various samples and were significantly correlated with the incidence of fruit infected on the trees at harvest and the incidence of postharvest brown rot. The best correlations (R2 = 0.82, 0.94, and 0.87 for total fruit infected on tree, brown rot in clusters, and postharvest brown rot, respectively, all significant at P < 0.001) were obtained with prune samples collected on July 30, 1996. Although some of the correlations of the incidence of latent infections and disease at harvest and/or postharvest were significant for samples collected in early June, late June, and mid July, the highest R2 obtained with samples collected at the end of July. This suggests that in years with conducive conditions to brown rot, sampling immature fruit at the end of July can be the best time for sampling fruit to predict brown rot at harvest and postharvest. In addition, this timing allows growers approximately a three-week period before harvest to decide on a pre-harvest spray. Therefore, at least for French prunes, we can predict fruit brown rot three weeks before commercial harvest by sampling immature fruit at the end of July.
In both 1996 and 1997, we chose three French prune orchards with expected different levels of brown rot and applied one or two preharvest fungicide sprays as described in the proposal. Among the three orchards used for the fungicide treatments, the one in Fresno County had 0% (1996) or 1% (1997) brown rot in the field at harvest and up to 9% (1996) or 5.1% (1997) after 14 days postharvest incubation. Therefore, none of the sprays were required in this orchard, but one spray was sufficient to reduce postharvest brown rot. In the orchard in Glenn County predicted to have medium level of brown rot in the field, at least one spray was necessary to reduce postharvest brown rot. Unfortunately, in 1996 the grower harvested the third orchard, which was expected to have high brown rot incidence in the field and postharvest before we were able to record the disease. But by harvesting 100 to 150 fruit per tree (leftovers) we were able to evaluate post-harvest brown rot. The results suggested that at least one fungicide spray was needed to reduce significantly the post-harvest disease. In 1997, because of the very unfavorable conditions for brown rot, the incidence of latent infections in prunes was not correlated significantly with either disease at harvest or postharvest. Again, in the orchard in Butte County expected to have high disease incidence in the field and postharvest, one pre-harvest spray was sufficient to reduce disease incidence at harvest in the field. One or two fungicide sprays resulted in similar levels of postharvest brown rot reduction (not differing significantly) in 1996, but in 1997 chlorine alone was an effective disinfectant. Very little brown rot developed and the postharvest treatments showed no effect in reducing disease whatsoever in fruit harvested from any of these three prune orchards.