Evaluating wild apples for resistance to postharvest diseases, understanding fungi causing decay in storage, and finding alternatives for controlling rots are briefly discussed.
My introduction in the world of tree fruit research was not out in the orchard, but in cold storages. When folks are opening up their storages to examine fruit bins this time of year, it's hard not to reflect on my roots. In 2010, I was fortunate enough to join a lab conducting postharvest disease research at the USDA-ARS in Beltsville, MD where a lot of exciting work was occurring. I was fascinated by learning apples being eaten in June were available year-round due to the ability of storing apples for nine months to a year in low temperatures and controlled atmosphere (a factoid I continually use to enlighten people) and apples are under the threat of disease during their entire existence, including while in storage.
Postharvest diseases caused by fungi are economically important and a consistent problem for the pome fruit and packing industry. Some of the most problematic diseases occurring in storage include blue mold (#1 offender), gray mold, bitter rot, Mucor rot, as well as common rot summer diseases (white rot, black rot, bitter rot). The fungi causing blue mold (Penicillium spp.) are especially important since these fungi are capable of producing harmful mycotoxins, such as patulin, that contaminate processed fruit products like juices, sauces, and butters. Resistant cultivars are available for problematic preharvest diseases, such as apple scab, fire blight, and bacterial spot; however, resistance to postharvest fungal diseases, like blue mold, is lacking in commercial cultivars since breeding has been focused on disease resistance for field production. Consequently, the need to figure out novel disease management strategies is necessary for staying one step ahead of these decay pathogens. Here, I want to share a few areas of research where I have been involved to give you an idea of what scientists are up to these days in fighting the war against postharvest diseases.
Evaluating wild apples from Kazakhstan for resistance to postharvest fruit diseases
Central Asia has been described to be the origin for the domestic apple. In the 1990s, samples from wild forest stands of the distant ancestor of the domesticated apple located in Kazakhstan were collected by USDA scientists that resulted in the "Kazak" collection of apple germplasm seedlings. The Kazak orchard is located at the USDA-ARS Plant Genetic Resources Unit on the campus of Cornell's New York Agricultural Research Station in Geneva, New York and represents a wide variety of horticultural traits, such as forms and colors. Consequently, the "Kazak Orchard" has been utilized to search for disease resistance, particularly to postharvest diseases. My former lab spent several years evaluating apple fruit from 83 different apple accessions for resistance to the most important apple storage diseases: blue mold (Penicillium expansum) and bitter rot (Colletotrichum acutatum). Two accessions were discovered to be "immune" to both diseases: despite apple wounds inoculated with spores, no disease developed. When we examined if the spores were still viable in the inoculated wound, the spores grew on artificial media indicating the apple was somehow shutting down the process of the spores' ability to germinate and cause infection. Such data provide a foundation for future research, such as serving as a source of resistance for conventional breeding programs and understanding the molecular mechanisms and genetics of resistance against postharvest diseases.
Evaluating fungi causing postharvest decay in storage
When managing decay pathogens in storage, it is important to determine which pathogens are responsible for causing decay and to evaluate the efficacy of the current fungicide chemistries controlling these diseases. Only three chemistries are available to control postharvest diseases: Scholar® (fludioxonil), Mertect® (thiabendazole), and Penbotec® (pyrimethanil). We obtained infected fruit from three different packinghouses using different preharvest and postharvest decay management practices, including organic. Over the course of two years, we consistently observed a variety of decay-causing fungi like Penicillium spp. (blue mold), Botrytis spp. (gray mold), Alternaria spp. (Alternaria rot), Mucor spp. (Mucor rot), Colletotrichum spp. (bitter rot), and Botryosphaeria spp. (white and black rots). As expected, the most prevalent fungi isolated were Pencillium spp. causing blue mold. In addition, we discovered several Penicillium spp. not previously described as pathogens of apple fruit. These species were especially important since both are capable of producing the patulin mycotoxin. Mertect® has been around a very long time and has well known documented resistance, especially for Penicillium spp. Not surprisingly, we found Penicillium spp. isolates from all three packinghouses that were resistant to Mertect®, with the lowest number of resistant isolates found on organic apples. The importance for the apple industry is that we now have specific information concerning the decay-causing fungi in storage, thereby allowing for the selection of the most appropriate chemical controls. (Special thanks to the State Horticultural Association of Pennsylvania for funding this work!)
Alternatives for controlling postharvest rots on stone fruit
The microbial community living on fruit is vast and some organisms can be beneficial to plants for controlling plant pathogens. Consequently, these microbes can be a source for finding alternatives to control diseases. Much research has been conducted evaluating the microbial community on pome and citrus fruits due to searching for biocontrol agents, some of which have become commercial products. Unfortunately, little is known about the microbial community on stone fruits and their ability to be exploited to control diseases. In collaboration with Dr. Wojciech Janisiewicz at the USDA-ARS Appalachian Fruit Research Station in Kearneysville, WV, we found the occurrence of yeasts on plum that were able to control brown rot on plums after harvest.& These results suggest that the surfaces of plums harbor yeasts that may be able to serve as a potential biocontrol for brown rot on stone fruits.
What can you do to prevent storage disease this season?
An article about diseases wouldn't be complete without management strategies included… Preventing disease in storage begins in the orchard: practicing orchard sanitation, properly timing fungicides, harvesting at optimum maturity, and using methods to prevent bruising. Development of disease is dependent upon the initial amount of inoculum in the orchard. The goal of sanitation is to reduce the number of fungal spores available to cause fruit rots for the coming season. Pre-season practices include removing and destroying infected plant tissue where fungal spores overwinter, such as mummified fruit (black rot, white rot, bitter rot, brown rot) and dead branches or twigs. Applying urea in the fall for scab management will also help knock back the spores of summer disease fruit rots that also infect leaves (black rot, bitter rot, Alternaria rot). During the season, it's important to be mindful of the fallen fruit underneath the trees. Fallen fruit encourages fungal spores residing in the soils (blue mold, gray mold, Mucor rot) to make copious amounts of spores, potentially contaminating bins in the orchard and thereby providing a vehicle for entering the packinghouse. Consequently, it's very important to minimize the soil on the bins during harvest time. This is especially important for controlling Mucor rot. Postharvest fungicides exist to control blue mold and gray mold in storage; however, no commercial fungicides are available to control Mucor rot. Although you may have done your best to minimize the amount of spores lurking around to cause fruit decay, fungicide applications are still important for cover sprays. Captan and Topsin® M are the workhorses for summer disease control, but strobilurins (Flint®, Merivon®, Pristine®) need to be considered, as well. Although strobilurins may have lost their efficacy to control apple scab as a result of resistance, strobilurins are still excellent controlling fruit rots and should not be overlooked when figuring out one's cover spray programs. In addition to controlling the typical summer diseases, the fungicides Merivon® and Pristine® are labeled to control blue mold and gray mold and are very helpful keeping disease in check after fruit are harvested. Although resistance to strobilurins has not been described or noted for the summer rots, resistance management strategies should still be followed, such as rotating and tank mixing with a broad spectrum fungicide.
Future postharvest disease research
Understanding your enemy (the pathogen itself) is the optimal strategy for effectively managing disease: what it is and how it works to determine where potential Achilles heels may be exploited. Two projects I'm currently pursuing in the lab address this strategy. Thanks to the generous funding from the State Horticultural Association of Pennsylvania, this summer and fall my lab will be evaluating decay pathogens occurring in Pennsylvania orchards during the season and comparing to fungal pathogens causing storage rots. In collaboration with my former lab at the USDA-ARS, I am finishing a project I had started during my tenure at the USDA: comparing how a weak blue mold pathogen modifies its host to cause infection compared to an aggressive blue mold pathogen. Needless to say, there will be a lot of exciting research to report in the not so distant future. So stay tuned for another "glimpse into the world of postharvest research!"