Crops and Diseases — Pome Fruit

Blue Mold

Blue mold is one of the most important postharvest diseases in pome fruit worldwide. Several Penicillium species are responsible for causing blue mold rot, but P. expansum is the most prevalent species. Economic losses due to the disease impact not only the fresh-fruit industry, but also to the fruit-processing industry, since some Penicillium species produce patulin, a mycotoxin that may affect the quality of apple juice.

Symptoms
Blue mold originates primarily from infection of wounds, such as stem punctures, bruises and mechanical damages from insects and birds on fruit (Fig. 1). Blue mold can also originate from infection at the stem and calyx of the fruit, but is less common than the wound infections. The decayed area appears a light tan to a dark brown. The decayed tissue is soft and watery. The lesion has a very sharp edge between diseased and healthy tissues, so decayed tissue can be easily separated from the healthy tissue (Fig. 2A2B). Blue or blue-green spore masses may appear on the decayed area, starting at the infection site. Decayed fruit has an earthy, musty odor. The presence of blue-green spore masses at the decayed area, and associated musty odor, are the positive diagnostic indication of blue mold (Fig. 3). Without the blue spore masses, it is not easy to differentiate blue mold from mucor rot (M. piriformis) that may produce a sweet odor rather than earthy odor.

Control
Thiabendazole (TBZ; Benzimidazoles) has been used for more than 40 years as either a pre-storage drench or a line spray to control the disease. However, Penicillium species developed resistance to TBZ quickly after the introduction of TBZ and failed to control blue mold. If a Benzimidazole fungicide is applied in the field, TBZ should not be used for postharvest treatment or vice versa, to avoid the risks of resistant development. Fludioxonil (FDL 230SC) and Pyrimethanil (Penbotec) are recently registered and highly effective to control blue mold, even the disease caused by TBZ resistant strains.

Sanitizing dump-tank and flume water is an essential practice to reduce infection of fruit by Penicillium spp. during the packing process. Fruit bins and storage rooms can harbor Penicillium spores including fungicide resistant strains. Bin and storage room sanitation should be implemented to break the circulation of resistant strains from packinghouses to orchards; thus, blue mold incidence for the following season can be reduced. Currently, TBZ, Fludioxonil, and Pyrimethanil can be applied by thermofogging and is proved as effective as drench treatments, without a potential contamination of decay and human pathogens.

Gray Mold

Gray mold, caused by Botrytis cinerea, is one of the most serious postharvest diseases in various crops worldwide, including apples and pears. This disease can cause significant losses during storage. When fruit is stored an extended period without a fungicide treatment prior to storage, losses as high as 60% can occur. This is because gray mold can spread from a decayed fruit to surrounding healthy fruit through fruit-to-fruit contact in storage bins.

Symptoms
Gray mold originates primarily from wound infections at harvest and during the postharvest handling process. Gala apples can easily get stem-bowl cracking that provides an avenue for B. cinerea to infect the fruit, particularly when fruit are over matured at harvest (Fig. 4). Stem-end gray mold is common on d’Anjou pears and also occurs on apples (Fig. 5). Calyx-end rots through latent infections during bloom and appears on apples and pears in the Pacific Northwest (Fig. 6).

The decayed area appears light brown to dark brown and the color is similar across the decayed area. The decayed area is firm or spongy. Decayed tissue is not separable from the healthy tissue, which can easily be differentiated from blue mold and mucor rot. Under humidity and warm conditions, fluffy white to gray mycelium and grayish spore masses may appear on the decayed area (Fig. 7), while little to no sporulation occurs in cold storage with a controlled atmosphere. The internal flesh of gray mold appears light brown to brown at the margin. Generally, gray mold does not have a distinct odor, but in advanced stages decayed fruit may have a cider-like odor, the entire decayed fruit may appear “baked”, and it can become soft like mucor rot or blue mold. Similar soft rot may occur when there is a secondary contamination by other fungal pathogens.

In the early stage of symptom development, it is difficult to differentiate gray mold from Sphaeropsis rot, Phacidiopycnis rot, Speck rot, and Bull’s-eye rot based on the symptomology. A simple diagnostic test can be performed by placing decayed fruit in a plastic bag with high humidity and incubate at room temperature for several days.

Control
A postharvest treatment with TBZ, fludioxonil or pyrimethanil applied prior to storage, is highly effective to control gray mold, particularly for those that originate from infection of wounds. B. cinerea resistance to TBZ is common, particularly where other benzimidazoles such as thiophanate-methyl are used in pre-harvest. Pre-harvest spray with effective fungicides near harvest can reduce gray mold in storage; however, it is not as effective as postharvest fungicide treatments. Spraying pre-harvest fungicides to control blossom infections is not ideal. Orchard sanitation by removing fallen fruit and debris may help reduce the inoculum level in the orchards.

Bull’s-Eye Rot

Bull’s-eye rot, caused by Neofabraea spp., is an important disease of apples and pears, particularly common in the U.S. Pacific Northwest. N. perennans and Cryptosporiopsis kienhozii are prevalent in Washington State, and N. alba is dominant in Oregon State and elsewhere including Europe, Chile, and Brazil. Bull’s-eye rot is commonly observed in Golden Delicious apples, particularly on apples from orchards with perennial canker problems on trees. Bull’s-eye rot also occurs in some other fruit-growing regions including Europe, Chile, and Australia.

Bull’s eye rot originates from infection at lenticels on the fruit skin, stem-end, and calyx-end. Fruit infections can occur any time during the fruit growing season, but fruit susceptibility increases as the growing season progresses. Since the infections occur mainly through water or rain splashes, overhead cooling or irrigation may increase the fruit infections. In the PNW, the disease is common on Golden Delicious apples and Bosc pears.

Symptoms
Bull’s-eye rot lesions are circular, flat to slightly sunken and appears light brown to dark brown with a lighter brown to tan center (Fig. 8). Decayed tissue is firm. Cream-colored spore masses in the aged decayed area may appear (Fig. 9). The symptoms are similar to that of bitter rot caused by Glomerella cingulata and side rot caused by Phialophora malorum. Since bitter rot is rare in the PNW, it is easy to differentiate bull’s-eye rot from bitter rot.

Control
Removing twigs and branches with cankers helps reduce inoculum level in the orchard. Recent efficacy trials conducted by WSU and Pace show that pre-harvest fungicide application failed to control bull’s-eye rot in storage. Postharvest fungicides, TBZ (TBZ 500D) and pyrimethanil (Penbotec) by either drench or thermofog, show consistent results and effectively controll bull’s-eye rot while fludioxonil and difenoconazole failed to show the efficacy against the disease.

Sphaeropsis Rot

Sphaeropsis rot, caused by S. pyriputrescens, is a recently identified postharvest disease of pears and apples. The disease was first observed in d’Anjou pears but causes more serious problems in apples. The disease is widely spread in the Pacific Northwest and British Columbia, Canada and was recently reported in New York State. Fruit infections occur any time during the fruit growing season, but decay symptoms do not develop at harvest and start to show 2-3 months after cold storage. Primary sources of inoculum for fruit infections are pycnidia produced on twig dieback and cankers on apple and crabapple trees. Pycnidia from dead bark or branches of pear trees are responsible for pear infections. Fruit infections start when the pycnidia splashed by any type of water, such as rainfall, overhead sprinklers and evaporative cooling, or high trajectory sprinklers from disease tissues to fruit. Fruit infected near harvest have higher incidence of Sphaeropsis rot in storage than early infections in the season.

Symptoms
Stem-end and calyx-end decays are most common, while infections through wounds or fruit lenticels are less common (Fig. 10A, 10B). The decayed area is firm or spongy with light tan to brown. In the advanced stage, pycnidia may form on the decayed area (Fig. 11). Sphaeropsis rot is very difficult to be diagnosed based on the symptomology due to the similarity with gray mold, speck rot, or Phacidiopycnis rot (Fig. 12A12B12C12D). Although it has a distinct odor like “used oil or bandage” from internal tissue of the decayed fruit, isolation of the fungus is highly recommended for accurate identification.

Control
Management of Sphaeropsis rot is based on orchard sanitation and fungicide application in pre- and/or postharvest. Removal of mummified fruit, twig dieback, cankers of apple and crabapple trees is very important to reduce inoculum levels in the orchard. ‘Manchurian’ crabapple that is extensively planted in commercial apple orchards in Washington State, is highly susceptible to the disease. Although several pre-harvest fungicides can reduce the decay incidence in storage, a postharvest fungicide application, including TBZ, pyrimethanil, or fludioxonil shortly after harvest is highly recommended. 

Speck Rot

Speck rot, caused by Phacidiopycnis washingtonensis, has been recently identified in the Pacific Northwest. The disease was mainly observed in apples but rare in pears. The disease was also reported in Europe and Chile. The fungus also causes fruit rot of persimmons in Italy and leaf blight disease of Pacific Madrones in western Washington. The epidemiology of the fungus is very similar to that of S. pyriputrescens causing Sphaeropsis rot of pome fruits. Infection on fruit can occur any time during the fruit growing season and increases as the season progress. Primary sources of inoculum for fruit infections are pycnidia formed on twig dieback and cankers on apple and crabapple trees. The pycnidial spores are spread by water-splashes and the infected fruit remains latent until 2-3 months of cold storage.

Symptoms

Stem-end and calyx-end rots are common in Red and Golden Delicious apples, while infections through fruit lenticels are common in Fuji (Fig. 13A , 13B). The decayed tissue is firm or spongy with light tan to brown, which is very similar to gray mold and Sphaeropsis rot. Brown to black specks with a white to light tan center may present on lenticels of red apple cultivars (Fig. 14); however, gray mold apples also have similar specks. In the advanced stage, decayed area becomes black, full of pycnidia that are easily stuck to the surface of cardboard apple trays (Fig. 15). Phacidiopycnis rot has a distinct odor, often not distinguishable from Sphaeropsis rot.

Control
Orchard sanitation is very important to reduce inoculum levels, which can be done by removing diseased plant tissues including mummified fruit, twig dieback, and cankers of apple and crabapple. Highly susceptible ‘Manchurian’ crabapples should be pruned before or right after bloom and replaced with less susceptible or resistant cultivars for a long-term management. Postharvest treatment with thiabendazole, pyrimethanil or fludioxoil is highly effective, while pre-harvest fungicide application near harvest can reduce the decay incidence in storage.

Mucor Rot

Mucor rot can cause serious economic losses on both apples and pears, particularly by a contamination through recirculating drenches. Several species of Mucor are responsible for the disease, but M. piriformis is most common. The disease appears less prevalent than blue mold or gray mold; however, fruit from the orchard with a previous history of mucor rot is consistently problematic. The fungus is a soilborne that survives as sporangiophores. Infested orchard soil attached to the bottom of fruit bins is the primary source for the contamination in drench solution or dump tank water.

Symptoms

Mucor rot originates primarily from infection of wounds on the skin of fruit, but stem-end and calyx-end Mucor rot is also seen on apples and pears. Infected tissues of pome fruit become soft and watery (Fig. 16). The decayed area appears light brown to brown with a sharp margin. The decayed tissue can be easily separated from the healthy tissue, which makes it difficult to differentiate from blue mold and other soft rots. Totaled fruit may produce alcoholic odor when kept in an airtight container. Gray columellate sporangia with black sporangiospores may appear on the decayed area (Fig. 17). Mucor rot fruit has a sweet odor. Without the signs of the pathogen present on decayed fruit, Mucor rot can be mistaken as blue mold, particularly in the early stage of symptom development.

Control
None of the fungicides currently registered for postharvest treatment of pome fruit in the US are effective against Mucor rot. Orchard sanitations are important to reduce the inoculum buildup and dispersal of the fungus by removing fallen and thinned fruit and avoiding flail mowing near and after harvest. Fruit should not be harvested in dry weather. Fruit bins should not be placed on wet, muddy soil. Over mature fruit are highly susceptible to mucor rot and other postharvest decays, particularly for ‘Gala’ apples that easily get stem-end cracking. Fruit pickers should not place fallen or dropped fruit on the orchard ground into fruit bins and should handle picked fruit carefully to minimize stem punctures. Thermofogging fungicides might be the best alternative to recirculating drench to avoid cross contamination of M. piriformis in drench solution. Sanitation for dump tank and flume water helps reduce mucor rot after packing.