By Mark A. Ritenour, Jiuxu Zhang, Liliana M. Cano and Megan M. Dewdney
The decay of fresh citrus fruit in Florida is usually caused by fungal pathogens that grow and develop in the hot and wet conditions typical to the state. While green and sometimes blue Penicillium molds and sour rot can cause Florida fruit to decay, the subtropical conditions of Florida favor diplodia (Lasiodiplodia theobromae) stem-end rot (SER), phomopsis SER (Diaporthe citri), anthracnose (Colletotrichum gloeosporioides) and, less frequently, brown rot (primarily Phytophthora palmivora and P. nicotianae).
Of these, diplodia SER is the most common postharvest decay in fresh Florida citrus. Although L. theobromae infects the fruit calyx primarily during the summer months, the infections usually remain dormant until after harvest, where careful handling, pre-cooling and the appropriate use of postharvest fungicides traditionally controlled postharvest disease development.
A PREHARVEST PROBLEM
However, after huanglongbing (HLB) became prevalent, diplodia SER substantially increased and is now observed on hanging fruit preharvest (Figure 1). This might be due to HLB-induced tree stress and related increases in the amount of weakened and/or dead wood in the trees, which promotes growth and sporulation of L. theobromae and subsequent fruit decay.
Traditional postharvest methods to control fruit decay become ineffective if it begins to develop in the fruit before harvest. Researchers at the U.S. Department of Agriculture demonstrated a strong relationship between HLB symptoms, fruit detachment force (abscission), ethylene production in the abscission zone, L. theobromae development before harvest, and ultimately, fruit decay. Improved methods to control fruit decay that can be employed in the field before the fruit is harvested are needed.
Preharvest application of effective fungicides could reduce fruit infection and colonization of fungal pathogens in the grove, also reducing postharvest decay. Preharvest fungicides such as Benlate (benomyl) and later Topsin (thiophanate-methyl) were once available and provided excellent decay control during harvest and postharvest handling and distribution. However, Benlate production was discontinued in 2001, and efforts to move Topsin from an EPA Section 18 emergency-use exemption to a full label were abandoned by the registrant in 2009.
There have been no known effective preharvest treatments with residual postharvest decay control for Florida citrus since then. Results from preharvest materials evaluated up through 2010 for their ability to control postharvest decay can be reviewed at journals.flvc.org/fshs/article/view/86240 and journals.flvc.org/fshs/article/view/83990. Alternative products need to be identified and registered for commercial use.
Studies during the 2019–20 and 2020–21 seasons evaluated the preharvest application of relatively new fungicides, a plant growth regulator (2,4-D) and an essential oil extract (thyme) for their ability to reduce postharvest decay on grapefruit (Table 1). Topsin was included to compare with the others as a likely “best case” control method.
The materials were sprayed on red grapefruit two or 14 days before harvest, and the fruit were exposed for 26 days to postharvest conditions that are known to promote diplodia SER development. The experiments were conducted six times using different red grapefruit blocks.
Figure 2 shows an example of commonly observed results. Topsin or Graduate A+ gave the greatest and most consistent reductions in fruit diplodia SER after harvest and storage. Graduate A+ was included because it is already registered for postharvest use and was effective in controlling postharvest diplodia SER if the decay did not start in the field.
Of the other materials, Headline and Miravis Prime occasionally reduced decay. Headline is the only material currently registered for preharvest use that at least gave occasional reductions in postharvest diplodia SER. Graduate A+ is not registered for preharvest use and is still rarely used postharvest in Florida because traditional postharvest fungicides are still effective.
This is unlike many other fresh citrus packing regions where pathogen resistance to the traditional fungicides required switching to new fungicides like Graduate A+. However, these studies used rates of Graduate A+ (fludioxonil + azoxystrobin) approved for postharvest use but that are higher than currently allowed in preharvest formulations utilizing these same fungicide components. Thus, future work will focus on evaluating combinations of fludioxonil, azoxystrobin and pyraclostrobin that will provide continuing postharvest decay control at rates allowed for preharvest use. Recommended use patterns will also be evaluated to minimize the development of fungal resistance.
LOOKING TO THE FUTURE
Continued research into better preharvest control methods is needed because of HLB’s ongoing impact on tree health and increasing postharvest decay. Work is also progressing to develop additional postharvest control methods such as chlorine dioxide gas-releasing sachets and biological products. Researchers are hopeful that further development of an integrated systems approach to controlling diplodia SER and other postharvest diseases will better assure decay-free citrus fruit from Florida.
Mark A. Ritenouris a professor, Jiuxu Zhang is a postdoctoral associate, and Liliana M. Canois an assistant professor — all at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Indian River Research and Education Center in Fort Pierce. Megan M. Dewdney is an associate professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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