By Achour Amiri and Kutay Ozturk, WSU Plant Pathology. Updated February 2020. Printable version Blue_Mold (2020.02.27) (F)
Blue mold is the most important postharvest disease of apples and pears worldwide and in the Pacific Northwest (Amiri and Ali, 2016; Rosenberger, 1990). Some strains of the blue mold causal fungus, Penicillium expansum, secrete patulin, a mycotoxin with mutagenic, neurotoxic, and gastrointestinal effects (Ramalingam et al., 2019). Therefore, blue mold is an economic concern not only to the fresh-fruit industry, but also to the fruit-processing industry since Penicillium contamination can result in patulin concentrations higher than the 50 µg/kg (ppb) limit in apple and pear processed products.
Blue mold of apples and pears is thought to be mainly caused by Penicillium expansum, although several other Penicillium species such as P. solitum, P. commune, P. verrucosum, P. chrysogenum and P. regulosum, have also been reported to cause blue mold decay. Although the growth of Penicillium spp. is limited in cold storage, the fungus can grow at temperatures as low as -3ºC (27ºF) and germination can occur at 0ºC (32ºF) (Rosenberger and Xiao, 2014).
Blue mold originates primarily from wounds, such as punctures, splits, bruises and limb frictions, infected by Penicillium spp. At early infection stages, blue mold symptoms include light tan to dark brown circular lesions with very sharp margin between diseased and healthy tissues (Fig 1A-C). The decayed tissue is medium-soft and watery. Decayed tissue can be readily separated from the healthy tissue, leaving a “bowl-like” cavity (Fig. 1D). Green spore masses may appear on the decayed area, starting at the infection site. As the decayed area ages, green spores may turn blue giving the common name of “blue mold” to the disease. Decayed fruit has an earthy, musty odor. Blue-green spore masses (Fig. 1G) on the lesion and the associated musty odor are diagnostic of blue mold. When decay has fully developed and the lesion softens, green-blue spore masses may be less apparent and blue mold can be misdiagnosed as Mucor rot. However, Mucor rot is characterized by a sweet odor and usually softens much quicker than blue mold (Table 1). Blue mold can also originate from infections through the stem bowl (Fig. 1E), where potential openings may have been caused by stem removal or through cracks and splits that are common on cultivars such as Gala and Honeycrisp. Calyx-end blue mold (Fig. 1F,I) may occur as a result of fruit drenching or in some cases due to core rot spreading to the surface of the fruit.
Figure 1. Symptoms and signs of blue mold caused by Penicillium spp., mainly P. expansum) on apples and pears.
Table 1. Disease comparison between blue and Mucor rot.
|Blue mold||Mucor rot|
|Texture||soft, watery||very soft, juicy|
|Color of decayed area||light tan to dark brown||light brown to brown|
|Signs of pathogen||white mycelium, blue or blue-green spore masses||gray mycelium with dark sporangia|
|Color of internal flesh||brown||light brown to brown|
Infection and Disease Cycle
Penicillium spp. are typical postharvest pathogens, and although some inoculum can be found in orchard soils and on organic debris on the orchard floor, fruit are seldom infected by Penicillium while on the tree. Penicillium survive easily as airborne spores in storage room air, walls, floors and bins. When bins are immersed in dump-tanks, large amounts of spores can be released into the water and carried out in flume water infecting additional fruit on the packing line. Infections may spread through extended storage, especially for packers who pre-size and store wet fruit (Amiri and Bompeix, 2005; Sanderson and Spotts, 1995). Infection by P. expansum may start immediately after harvest on fresh wounds caused during picking and handling (Amiri and Bompeix, 2005). These lesions will develop slowly in cold storage and may produce large amounts of spores that are air-disseminated by continuous fan movements during storage to infect additional fruit, air, walls and bins. Infections by Penicillium occurring through lenticels are rare but possible, especially as the fruit over-mature and lenticels may breakdown. On rare occasions, remaining infected parts of flowers can serve as a source of inoculum of Penicillium and other pathogens to infect seeds inside the fruit that may cause “core rot” later in storage. Although certain cultivars such as Honeycrisp and Gala may seem more susceptible to blue mold infections, all current commercial apple and pear cultivars are susceptible to blue mold, especially through wounds.
Orchard sanitation to remove decayed fruit and organic debris on the orchard floor helps reduce inoculum levels of Penicillium spp. in the orchard. Good harvest and handling practices designed to minimize fruit punctures and bruises are critical to minimize blue mold infections.
However, because postharvest environments create more favorable conditions for infections and decay development, mainly due to high humidity levels, postharvest sanitation practices have a greater impact on blue mold control. It is important to clean and sanitize rooms (air, walls, floors) and bins at least once a season before using them to store a new crop. Sanitation of the packing-line from the dump-tank all-the-way to the sorters needs to be done regularly during the packing season. Several sanitizers, i.e. chlorine and chlorine dioxide, hydrogen peroxide, organic aids, electrolyzed water and ozone, are available and all have different efficacies and uses (Bernat et al., 2018; Feliziani et al., 2016). It is recommended to consult extension specialists or consultants in your area and the product labels to optimize sanitation. Recent cold or hot fogging technologies have shown good levels of efficacy in sanitizing larger facilities (entire rooms, lines, stacked bins) more efficiently.
Most current preharvest fungicides applied days before harvest may not provide the highest level of efficacy as Penicillium infections typically occur after harvest. Therefore, postharvest fungicide applications remain the most effective chemical way to control blue mold. Currently, there are four single-site postharvest fungicides labeled for postharvest application (Table 2). Thiabendazole (TBZ, Mertect) has been largely used in the PNW since the 1970s until early 2000 when two new postharvest fungicides, fludioxonil (FDL, Scholar) and pyrimethanil (PYR, Penbotec), were registered for postharvest decay management. In 2016, difenoconazole (DIF, Academy) was labeled for postharvest application in pome fruit and other commodities. With current absence of resistant strains these four fungicides have a high level of efficacy against blue mold, with FDL shown to have the highest efficacy. For decades, postharvest fungicides have been applied as a drench at harvest or as a spray on the packing-line. However, in recent years, thermonebulization (TNB, also called fog or aerosol or dry application) has become common in the PNW pome fruit industry. One of the advantages of TNB is that it reduces the spread of spores between bins compared to when non-sanitized bins are used for drenching. Although TNB still requires more optimization for other diseases, it has been shown to have a better efficacy against blue mold compared to drenching. Formulations to apply TBZ, PYR, and FDL through TNB exist already, whereas DIF’s dry formulation is pending. Besides these four single-site fungicides, captan, a multi-site fungicide, is also registered for postharvest application. Its efficacy is fair and may help for short-term storage and for managing fungicide resistance development. Postharvest fungicides should be applied soon after harvest because the more time that elapses after harvest, the higher the risk for decay to develop in storage.
Fungicide Resistance Management
Penicillium expansum is considered to have a medium to high risk for fungicide resistance development. Because the fungus sporulates profusely, there is a risk that many spores will be exposed to the applied fungicide which increases the risk of selecting for resistance. Resistance to TBZ has been reported in the PNW (Li and Xiao 2008) and elsewhere. In Washington State, frequencies vary between packers based on the usage frequency. Resistance to PYR (Penbotec) has been found to range between 0 and 50% in Washington (Caiazzo et al., 2014, Amiri and Pandit, 2019) whereas 5% to 10% of the population showed tolerance to FDL (Scholar) (Amiri and Pandit 2019). Given the known risk and the limited number of postharvest fungicides available, it is critical that these materials are used appropriately to limit the development of fungicide resistance. Knowing what type and level of resistance in each warehouse is the first step in developing an effective spray program. WSU researchers (Contact Dr. Amiri) can help with conducting a risk assessment. The second step is to implement sound annual cleaning and sanitation to remove or reduce the residual inoculum from previous seasons. It is critical to apply a different FRAC group for postharvest treatments each season. Although resistance to TBZ has been documented, 50% of PNW packers can still use this fungicide if they follow good sanitation practices. It is recommended not to apply TBZ (FRAC 1) postharvest if Topsin-M (FRAC1) was used preharvest. The two most effective fungicides PYR (FRAC 9) and FDL (FRAC 12) can be alternated on a yearly basis. In some cultivars, known to be susceptible to bull’s eye rot (BER), is recommended to apply FDL with TBZ to improve control of BER. Academy (DIF + FDL) has shown high efficacy levels against blue mold after seven months of cold storage (Ali and Amiri 2018; Jurick et al. 2018). In Washington State, Academy can be applied via drench at harvest given that packers follow specific fungicide waste management practices (https://agr.wa.gov/wastepesticide). A formulation for a dry application of Academy is under development. For other states and regions, please contact your local agencies for additional information. For organic packers, there are not many options to fight blue mold postharvest. Sanitation is more critical when growing, storing and packing organic fruit. A recent study has shown that a combination of peroxy-acetic acid and hydrogen peroxide applied at 50 fl oz/100 gal via dip was effective in controlling several postharvest diseases.
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Dr. Achour Amiri,
Tree Fruit Research and Extension Center