Botrytis Gray Mold as a Postharvest Pathogen in D'Anjou Pear
Although researchers are aware of the specific organisms responsible for postharvest decay in the pear cultivar d'Anjou, there is little documented information on the relative importance of decay organisms in relation to each other and the levels of decay they are each responsible for. A study was made of the incidence of decay after storage in d'Anjou pear for the 1994 through 1996 crops, as detected on the cull lines of three packinghouses in the Hood River area of Oregon.
Results indicated that Botrytis cinerea was responsible for 55% of the total decay, whereas Penicillium spp., and Mucor spp. were responsible for 24 and 8%, respectively. The remaining 13% was ascribed to other pathogens including Cladosporium, Alternaria and Pezicula (bull's eye rot).
Of the decay caused by Botrytis cinerea, 26% of infections were seen to originate at puncture or wound sites whereas 18% of infections were caused by Penicillium spp. at wounds. These figures indicate that wounds resulting from handling of the fruit at harvest and prior to storage are important sites of infection for both Botrytis cinerea and Penicillium spp.
Stem-end decay caused by Botrytis cinerea was found in 20% of the fruit, whereas calyx end decay caused by this fungus was observed on 8% of the fruit. Penicillium spp. were responsible for 4% of the fruit showing stem-end decay and 3% calyx-end decay.
These results indicate that Botrytis cinerea is the major pathogen involved in postharvest decay of d'Anjou pears in the Hood River area, and that stem-end infections cause significantly higher levels of decay than calyx end infections.
Drenching Effect on Decay in Stored D'Anjou Pears
One of the ways which several packinghouses employ to reduce the incidence of gray mold and blue mold in stored d'Anjou pears is the application of the benzimidazole fungicide thiabendazole (TBZ) as a prestorage drench. In this study we made a comparison of decay incidence in culled fruit of nondrenched and TBZ drenched d'Anjou pears. The drenched fruit showed a decrease in incidence of decay caused by Botrytis, but a marked increase in decay caused by Penicillium species at puncture or wound sites on the fruit. This study did not examine the influence of the drench on the overall decay levels in the lots examined, but focused on the relative incidence of specific decays. A study which needs urgent attention is the comparison of overall decay levels in drenched and nondrenched fruit from the same fruit lots.
The benzimidazoles, e.g., TBZ, are a group of highly effective broad spectrum (active against many pathogens) fungicides (Russel, 1995) which act by inhibition of tubulin biosynthesis (Davidse, 1973), a process under simple genetic control with frequent mutation to resistance (van Tuyl, 1975 as cited by Russel, 1995). In fungi with short life cycles, like Botrytis and Penicillium, the shift to resistance can happen very rapidly. Spotts and Cervantes (1986) found approximately 25% of Penicillium spores in dump tank water are resistant to the benzimidazole benomyl; however, after a single exposure to benomyl in a drench, 80% of the spores were resistant to this fungicide.
History has shown that when the benzimidazoles are used extensively against Botrytis cinerea in the orchard, vineyard or field situation, resistance soon appears. Loss of control in vineyards in Northern France appeared after only 3-4 seasons of use (Leroux and Clerjeau, 1985). Resistant strains tend to be as fit as sensitive strains and will survive in the population even after fungicide use is stopped (Leroux and Clerjeau, 1985). Currently TBZ is the only really effective fungicide available to the pome fruit packing industry for control of gray mold, and every effort needs to be made to protect this important tool. The use of all the benzimidazoles (as they all have the same mode of action) must be limited to a single application in the packinghouse, and any use of this group of fungicides, including Topsin, in the orchard on any of the pome fruit crops should be strongly discouraged.
Timing of Botrytis Infection
Botrytis cinerea is a pathogen of over 200 plant hosts (Jarvis, 1980). This fungus has been isolated from aerial surfaces of a wide range of plants including those that are healthy or senescing. It can also survive and multiply in this saprophytic form on plant debris, including decaying leaves and fruit.
Now that we have evidence as to which organisms are responsible for decay in d'Anjou pears, and their relative importance, we need to determine when infection of the fruit occurs. It is widely assumed that infections resulting in calyx end decay occur at or around full bloom, and consequently measures aimed at controlling calyx-end rot caused by Botrytis involve the use of fungicides shortly before to shortly after full bloom. To determine if and when calyx-end and stem-end infections occur, fruit from trees not sprayed with fungicides were collected every two weeks from full bloom until harvest, and the stamens, pistils, sepals and stems of blossoms were plated out on an agar medium semi-selective for Botrytis cinerea (Kerssies, 1990).
Results indicate that calyx infections show two distinct peaks, in mid to late May (i.e. about a month after full bloom) and just prior to the final sample date. Thus the application of an effective anti-botrytis fungicide near full bloom should reduce the level of calyx infection in the orchard. It can be generalized that there are two peaks in stem infections, however the levels of infection are much lower than those found in the calyxes.
In this experiment, fruit was also harvested at commercial harvest time and placed into storage. This fruit will be evaluated for gray mold decay after storage for three, six and eight months, and the relationship, if any, between levels of infection detected in blossoms and preharvest fruit to that in storage will be determined. As yet the fruit in storage has not been evaluated for decay.
As Botrytis cinerea is able to survive and multiply on almost any of the Dicotyledons, and many of the Monocotyledons (Jarvis, 1980), keeping the orchard cover mowed short and free of flowers during full bloom of the pears may be an important and effective way of reducing the botrytis inoculum levels in the orchard. Any decaying fruit from the previous crop should also be removed before full bloom.
Timing of Infection Resulting in Stem-End Decay
The short fleshy stems of d'Anjou pears appear to be highly susceptible to infection and decay by Botrytis. Whether orchard infections prior to harvest are responsible for the levels of stem-end decay observed in stored d'Anjou fruit is not clear. As stem end decay by Botrytis cinerea has been shown to be responsible for greater levels of postharvest decay, determining when stem end infections occur has become a research priority.
Fruit from 14 commercial d'Anjou orchards was collected once a month from full bloom until harvest. The stems were dissected out and plated onto an agar medium semi-selective for Botrytis cinerea. Records were made of stems infected with this fungus. At commercial harvest times, fruit was sampled from each of the fourteen orchards, and placed into storage. This fruit will be evaluated for decay after three and five months, and the relationship, if any, between levels of infection detected in preharvest and at harvest fruit stems to that in storage will be determined. As yet the fruit in storage has not been evaluated for decay. Results of agar plating of stem tissue indicate that fruit sampled up until July show no stem infections by Botrytis cinerea. Stem infections increased slightly as the fruit matured, indicating an increased susceptibility of the stem tissue. It would appear that the majority of stem end infections occur once the fruit has been harvested, and contamination of the abscission zone of the stem of harvested fruit by Botrytis spores present in the air, on fruit surfaces or bin surfaces could be an important source of infection resulting in stem-end decay.
A Predictive Model for Gray Mold Decay in Storage
Levels of gray mold decay in stored d'Anjou pears vary from year to year, grower to grower, and often fruit lot to fruit lot of individual grower. The availability of a reliable model for predicting gray mold decay would be of great value to the industry, as fruit predicted as having a high incidence of gray mold could be sold first, whereas fruit with low incidence of gray mold could be safely stored for marketing later.
Currently we are doing experiments to determine whether there is any relationship between levels of Botrytis infections before harvest, as detected by plating out of calyx and stem components on semi selective media, and levels of gray mold decay pears held in cold storage for up to eight months.
In a study done in two orchards in 1995-96, stem end decay of d'Anjou pear fruit in cold storage increased from early January to May. However, the amount of fruit infection in May (6.1 and 3.7% for orchards A and B, respectively) was less than half the potential level of infection indicated by the percent of stems infected with Botrytis cinerea before harvest (16.7 and 10.0% for orchards A and B, respectively). In this experiment, infection of stem tissue just before harvest and the populations of B. cinerea in orchard A soil exceeded those of orchard B. These higher levels of B. cinerea in the soil appear to be correlated with the higher percent of stem end gray mold in stored fruit from orchard A.
These findings led us to take a closer look at potential sources of inoculum for Botrytis infection, and we are currently investigating the relationship between levels of Botrytis cinerea at a number of important stages in pear production and gray mold decay levels found in stored fruit. Some of the potential sources of inoculum being investigated are the air, soil and soil litter in orchards throughout the season; fruit surfaces at harvest; the dump tank and packingline water; and the air in cold storage rooms and in the packingline environment. Inoculation studies with dry spores are in progress and are designed to establish the relationship between spore levels in the air and on fruit surfaces to fruit decay in storage. These findings could well contribute to a greater understanding of the gray mold decay problem in pears and lead to the development of a reliable model for predicting gray mold in storage.
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