With 90,000 tons produced a year, Florida is the second-leading grower of strawberries in the United States. Strawberries in Florida are typically transplanted from locations such as California and Canada. They are most susceptible to disease when they are first transplanted, according to Dr. Marcus Marin, an assistant scientist at the UF/IFAS Gulf Coast Research and Education Center.

Marin, 2022 Plant Pathology Ph.D. graduate, works to protect the sweet red fruit. Marin and a team of researchers have identified and developed a test for a gene responsible for fungicide resistance in a disease that threatens the important crop.

Growers typically use a fungicide called mefenoxam to treat the diseased plants. “[Mefenoxam] has been the gold standard here in Florida for a long time,” Marin said.

Phytophthora crown rot, typically caused by the pathogen Phytophthora cactorum, poses one of the biggest disease threats to strawberries. Characterized by wilting, stunting and plant collapse, the disease can dramatically affect yield or even lead to the loss of plants, according to Marin.

Around 2016 or 2017, growers and researchers started noticing fungicide resistance in fields affected by phytophthora crown rot. If the plants are resistant to a fungicide, growers are going to expend resources applying it to no avail. “So the plants are still collapsing, and the disease is still progressing, and it’s a waste of money and time,” said Marin.

Growers need to know what pathogen is infecting the plant, and they need to know if that strain of the pathogen is sensitive to the fungicides they are using to treat the disease. “We studied the genetics of the Phytophthora strains recovered from these plants. By doing whole genome sequencing and really looking at the DNA, we were able to find and pinpoint genes that are present in the resistant strains; those genes may be the cause of the fungicide resistance.” Identifying the genes allowed Marin and his colleagues to identify fungicide-resistant strains by designing polymerase chain reaction PCR  tests, “which everyone knows about now because of  COVID,” Marin said.

Marin’s mefenoxam resistance research was part of his Ph.D. research into Phytopthora diseases in strawberries, for which he received the UF/IFAS Award of Excellence for Graduate Research – Best Dissertation in Agricultural Systems, and Overall Best UF/IFAS Dissertation last year.

Marin set up the new testing system at the plant diagnostic clinic at GCREC, which he coordinates with Dr. Natalia Peres. “We run the PCR test on symptomatic samples we receive at the clinic,” Marin said. “Within 24 hours, we can let growers know what pathogens they have and whether they are resistant or sensitive to mefenoxam.” Before the development of the PCR test, determining fungicide resistance could take as long as two weeks. Symptomatic plants had to be cultured, and then tested for growth in the presence of the fungicide, he said. “By addressing the issue right away, we can minimize the disease development, so there’s still a chance of saving the field.”

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