Sooty blotch and flyspeck (SBFS) degrades the cosmetic appearance of fresh market apples. Apples that are heavily infected with SBFS are typically diverted to processing use instead of the more profitable fresh market sale, resulting in large economic losses to growers.
SBFS is a disease complex caused by more than 80 species of fungi. Different SBFS species result in a range of different types of colonies on the apple surface - for example, some look like dark smudges, whereas as other look like clusters of tiny black dots. Unlike other plant pathogens, SBFS fungi colonize only the outer waxy layer and cuticle of the fruit without damaging living cells. SBFS fungi feed on nutrients that leach out of maturing apples, as well on substances they can absorb from dissolving the wax and cuticle on the fruit surface by using enzymes.
SBFS fungi can colonize the surface waxy layer of hundreds of species of plants, including pear, papaya, plum, and banana as well as raspberry (Gleason et al., 2011). They often grow on the waxy outer layer of canes, leaves, and branches as well as fruits. In the eastern half of the U.S., some plants, such as blackberry live near the borders of apple orchards, where they can act as “reservoir hosts” by providing spores that infect the apples.
Microscopic-size spores from SBFS fungi on the reservoir hosts spread by wind and rain to nearby orchards. These fungi can also survive the winter on the reservoir hosts and cause SBFS infection in subsequent years. SBFS signs usually appear late in summer as the fruit mature, and are especially severe when the weather during the growing season has been relatively wet. Early-maturing apple varieties are usually free from SBFS infections, apparently because the infections are not old enough to cause visible colonies by the time the fruit are harvested (Biggs et al., 2010).
Small-scale apple growers often thrive on a direct-to-consumer approach, where creating positive relationships with customers helps to insure repeat business. Some of the common marketing channels for small-scale growers are:
The most widely used SBFS management strategy relies on fungicide sprays. Since SBFS spores can colonize fruit at any growth stage, most growers apply fungicide sprays during the summer on a calendar-based system, in which sprays are applied at pre-scheduled intervals (weather permitting) of 10 to 14 days from several days after the flowering period ends until harvest. A disadvantage of the calendar-based approach is that it may sometimes result in spraying when there is no threat of an SBFS outbreak; in other words, it can waste money as well as the grower’s time. Another reason to spray only when necessary is concern about environmental and human health hazards associated with some of the fungicides that are widely used against SBFS.
Warning systems were developed to spray fungicides more efficiently: that is, only when weather conditions posed a high risk of an SBFS outbreak. An SBFS warning system for the Midwest is based on relative humidity (RH) (Duttweiler et al., 2008). Beginning about 10 days after the flower petals fall off, growers can wait until a total of 385 hours with RH above 90% have accumulated before applying the next spray. Depending on the growing season, the grower can save as few as one or as many as five sprays per season by using the warning system, without adding any risk of fruit damage due to SBFS. Each spray saved is equivalent to saving about $50 per acre when all the costs, including labor and machinery as well as the spray itself, are calculated.
Some apple growers are hesitant to adopt the SBFS warning system. Using the system requires a data logger and sensor (about $500) to record the RH data each hour, plus a laptop computer or other mobile device to download the data from the data logger and run the disease model at weekly intervals. In other words, there are significant costs, time demands, and a learning curve associated with transitioning from calendar-based fungicide spray timing to a warning system.
In addition to disease-warning systems, other cultural practices that can help to reduce the risk of SBFS outbreaks include:
The first author was awarded an Academic Staff Training Scheme scholarship by Universiti Sains Malaysia and Ministry of Higher Education Malaysia for his Ph.D. program at Iowa State University. We thank Dr. Leonor Leandro and Dr. Nicholas Peters from Iowa State University for providing access to their Micro 456 (Principles of Mycology) and PLP 408 (Principles of Plant Pathology) classes, respectively, and to the students in these classes for their suggestions for improving the case study. We also thank Jiani Chen for her help in structuring the case study.
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