Photo by Mark Ritenour
By Megan M. Dewdney, Eva L. Mulandesa and Mark A. Ritenour
Greasy-green began to be reported as a problem in Florida by fresh fruit packers and growers around 2020. The disorder, with green blotches on the grapefruit surface, appeared to be different from other known rind disorders. The effect was costly for growers and packers, with reported losses between $20,000 and $1.7 million per year. The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) was asked to determine what was causing this stubborn green color and whether postharvest practices could be modified to reduce this problem and improve packout and profits.
The greasy-green disorder was somewhat mysterious. It could sometimes be seen in the field, but it was most frequently observed once fruit were degreened. In the field, the lesions looked similar to early rind blotch because one of the symptoms is a stubborn green color on the rind. However, it was not identical as there was no associated dark blue-black stippling and there were no greasy spot lesions on leaves in these groves. The disorder appeared even though greasy spot/rind blotch management was nearly optimized. The question changed to whether greasy-green was a new form of the disease or if it was an entirely new disorder.
UF/IFAS worked to answer the question whether the fungus Zasmidium citri-griseum, responsible for greasy spot and rind blotch, was also associated with greasy-green symptoms. Researchers collected samples from the grove, looked at leaf litter, isolated the fungus and confirmed its identity. Then they collected fruit for postharvest experiments and sampled fruit lesions to determine if the fungus was present on the lesion surface.
GETTING ANSWERS
Researchers collected leaf litter samples from greasy-green affected grapefruit groves and from other groves with symptomatic greasy spot to determine if the same organism was present in both samples. The organisms were the same, but it was approximately 10 times harder to collect isolates from the grapefruit groves where the leaves were symptomless. This was answer 1: The same organism was present in both types of groves, but the frequency of isolation was much lower in greasy-green groves. Researchers did not directly measure how many spores were present, but since this fungus produces nearly all its spores from the leaf litter, it indicates that the inoculum level was likely lower in the greasy-green affected groves.
Fruit were collected just before harvest for several seasons, and multiple degreening experiments were conducted to see if changing the degreening conditions would remove the greasy-green symptoms from the fruit. Red and white grapefruit were degreened from a commercial grove at 85°F for up to five days. Another test exposed the fruit to 37°F (cold shock) for one day before degreening for a day. While degreening fruit with 5 parts per million ethylene for up to five days significantly improved peel color compared to the control, in all studies, fruit with greasy-green symptoms never attained satisfactory peel color. This was answer 2: Greasy-green cannot be adequately remediated by postharvest degreening treatments.
The final piece of the puzzle was indirect evidence that Z. citri-griseum was causing the greasy-green symptoms. It is extremely difficult to isolate the fungus from heavily symptomatic fruit. Trying to isolate the fungus from fruit where there are very mild symptoms which may or may not be caused by the fungus would require extremely good luck and may not be repeatable. Repeatability is very important when establishing a cause. Since isolation was not a viable option, researchers developed molecular tools to work with this understudied organism.
Once the tools were developed, the surface of the fruit was painted with clear nail polish in the symptomatic areas. Using a microscope, a fungus was observed growing on the surface, with more growth on areas with symptoms. Some of the peels were also tested molecularly for specific identification of the fungus. This gave answer 3: The same organism found in the leaf litter was also present on the surface of the grapefruit.
This result was consistent with what is known about the biology of the fungus. It mostly grows on the surface of fruit and leaves. Only in the fall does it start to penetrate plant tissues through the stomates. Researchers followed how much fungal growth occurred on leaves and fruit throughout the season and found increasing amounts in the second half of the summer and into the fall in a separate set of experiments.
RESEARCH CONCLUSIONS
What can be concluded from this work is that the greasy-green disorder is caused by the same fungus that causes greasy spot and rind blotch. It is not 100% clear why the symptoms are different, but researchers suspect that it is because there is only a small amount of inoculum present in the groves, because there are no leaf symptoms. Although there is not enough infection to cause visible symptoms, it is enough to form spores that infect the fruit at low levels.
Because the color of fruit after harvest cannot be substantially improved, greasy-green needs to be managed in the grove. Copper is not suppressing the fungus completely, so well-timed applications in late May and mid-June should be applied. At least one mode of action other than copper should be included in the rotation. The current fungicide recommendations are found in the Florida Citrus Production Guide.
Megan M. Dewdney and Mark A. Ritenour are UF/IFAS professors, and Eva L. Mulandesa was a UF/IFAS postdoctoral research associate.
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