Gray mold is a common postharvest disease on apples and pears wherever these fruits are grown worldwide. This disease can cause significant losses on both apples and pears during storage. Losses as high as 20-60% due to gray mold are not uncommon after an extended period of storage, particularly on fruit that were not treated with fungicides prior to storage, because gray mold has the ability to spread from decayed fruit to surrounding healthy fruit through fruit-to-fruit contact during storage.
Gray mold originates primarily from infection of wounds such as punctures and bruises that are created at harvest and during the postharvest handling process. Gala apple fruit at harvest tend to split at the stem bowl area, which provides an avenue for B. cinerea to infect the fruit. Stem-end gray mold is common on d’Anjou pears and also occurs on apples (Fig. 2). Calyx-end gray mold has been observed on pears in the Pacific Northwest but generally is not very common.
The decayed area appears light brown to dark brown and color is similar across the decayed area. The decayed area is spongy, and diseased tissue is not separable from the healthy tissue, which is different from blue mold (a soft decay) (see the comparison in Table 1). Under high relative humidity conditions, fluffy white to gray mycelium and grayish spore masses may appear on the decayed area. The internal decayed flesh appears light brown to brown at the margin. Generally, gray mold does not have a distinct odor, but in advanced stages decayed apples may have a “cedar-like” smell. In advanced stages, the entire decayed fruit may appear “baked” and eventually may turn softer than in the early stage.
In the early stage of symptom development, gray mold and Phacidiopycnis rot on d’Anjou pears are very similar, but the fruit flesh at the margin of Phacidiopycnis rot appears translucent and water-soaked, whereas internal decayed flesh of gray mold usually appears brown (see Table 2 for comparison).
The causal agent of gray mold is Botrytis cinerea Pers., teleomorph Botryotinia fuckeliana (de Bary) Whetzel. Isolates recovered from decayed apple and pear fruit vary greatly in sporulation on potato dextrose agar. Some isolates sporulate abundantly, whereas others produce only sclerotia or sclerotia with scarce conidia. Mycelium of B. cinerea grows at temperatures as low as -2ºC. Conidia can germinate at 0ºC.
Botrytis cinerea colonizes organic matter in the orchard. Conidia of the fungus are dispersed mainly by air currents and water splash. Gray mold of apples and pears primarily originates from infection of wounds that are created at harvest and during the handling processes. B. cinerea can infect stems of d’Anjou pear fruit and causes stem-end gray mold. d’Anjou pears have thick stems. At harvest, pear fruit are detached from the trees and fresh wounds at the stem are exposed to potential contamination by B. cinerea. B. cinerea is able to grow at pear storage temperatures of -0.5 to 0ºC, move through the stem of the fruit to reach the fruit flesh after a period of time in cold storage, and then cause decay. The fungus can also invade stems of Delicious apples and cause stem-end rot. On pears, it has been shown that incidence of gray mold is correlated with spore levels on the fruit surface of d’Anjou pear fruit. B. cinerea has been reported to invade floral parts of pear fruit during bloom, causing calyx-end rot during storage, but calyx-end gray mold is not very common on apples and pears grown in the semi-arid climate of eastern Washington State.
Secondary infection of fruit through fruit-to-fruit contact during storage is commonly seen after a long period of storage and can cause significant losses. Mycelial growth and secondary infection through fruit-to-fruit spread is enhanced by high relative humidity. Fruit infected by gray mold due to secondary infection in storage bins may not have visible symptoms or lesions are very small at the time of packing, and thus infected fruit may be packed but symptoms develop on packed fruit during storage or transit.
Orchard sanitation to remove decayed fruit and organic debris on the orchard floor helps reduce inoculum levels of B. cinerea in the orchard. Good harvest management to minimize punctures and bruises on the fruit skin helps avoid decay from wound infections. Gala apple fruit tend to split at harvest. Harvesting Galas at the right maturity to minimize splits at the stem bowl would help avoid gray mold from infection at that site.
Preharvest fungicides such as thiram and ziram applied near harvest provide some control of gray mold. Research is in progress to evaluate more effective fungicides as preharvest treatments for control of postharvest gray mold. The vast majority of B. cinerea isolates from apple-related sources in Washington State are sensitive to thiabendazole. A postharvest drench treatment with Mertect (thiabendazole) applied prior to storage is effective to control gray mold, particularly for those that originate from infection of wounds. In 2004, two new postharvest fungicides, Penbotec (pyrimethanil) and Scholar (fludioxonil), were registered as postharvest treatments for control of postharvest diseases of pome fruits in the U.S. Both fungicides are labeled for use as either drench treatments or online sprays. It has been shown that both fungicides applied as prestorage treatments are effective to control gray mold from wound infections.
Photo Plate: Gray Mold
Fig. 2. Symptoms and signs of gray mold (Botrytis cinerea) on apples and pears.
|A: Gray mold originating from infection at the stem-bowl crack of a Gala fruit; decayed area brown, spongy to firm
||B: Early stage of gray mold originating from fruit-to-fruit spread
||C: Advanced stage of gray mold; white to gray mycelium covering the decayed area under high humidity
|D: Gray mold originating from infection at stem or stem bowl; gray spore masses at the stem bowl area
||E: Gray mold originating from wound infection on a d'Anjou pear fruit
||F: Stem-end gray mold commonly seen on d'Anjou pears; white to gray mycelium
|G: Sclerotia of B. cinerea formed on the surface of a decayed d'Anjou fruit at advanced stage
||H: Gray mold originating from calyx infection of a d'Anjou fruit
||I: Nesting of gray mold due to fruit-to fruit spread during storage; white to gray mycelium
Table 1. Comparison between gray mold and blue mold
|Characteristics||Gray mold||Blue mold
|Texture||spongy or firm; decayed tissue not separable from the healthy tissue||soft, watery; lesion with a sharp margin; decayed tissue completely separable from the healthy tissue, leaving it like a “bowl”
|Color of decayed area||light brown to dark brown||light tan to dark brown
|Signs of pathogen||fluffy white to gray mycelia; sporulation under high humidity; gray to brown spore masses; black sclerotia may form||white mycelia and blue or blue-green spore masses; sporulation often starts at the infection sites (wounds)
|Color of internal flesh||light brown to brown||brown
|Odor||generally not detectable||earthy, musty
Table 2.Comparison between gray mold and Phacidiopycnis rot on pears
|Characteristics||Gray mold||Phacidiopycnis rot
|Texture||spongy or firm; decayed tissue not separable from the healthy tissue||spongy; decayed tissue not separable from the healthy tissue
|Color of decayed area||light brown to dark brown; color similar across the decayed area||initially watersoaked, then light brown to brown, later black; color varies with age
|Signs of pathogen||fluffy white to gray mycelia; sporulation under high humidity; sclerotia may form||white mycelia under high humidity; black pycnidia form on decayed fruit at advanced stages but often are immature under commercial cold-storage conditions
|Color of internal flesh||light brown to brown at the margin||translucent, clear at the margin
|Odor||generally not detectable||mild, distinct
Coley-Smith, J. R., Verhoeff, K., and Jarvis, W. R. 1980. The Biology of Botrytis. Academic Press, 318pp.
Lennox, C. L., Spotts, R. A., and Cervantes, L. A. 2003. Populations of Botrytis cinerea and Penicillium spp. on pear fruit, and in orchards and packinghouses, and their relationship to postharvest decay. Plant Dis. 87:639-644.
Sommer, N. F. 1985. Role of controlled environments in suppression of postharvest diseases. Can. J. Plant Pathol. 7:331-339.