Oct 18, 2019
Tomato bacterial spot fought in transplant seedlings

New findings from University of Florida (UF/IFAS) scientists could help tomato growers nationwide in their battle against a damaging disease.

With 37,000 acres, Florida’s tomato industry is a $520 million-a-year crop, so it’s critical to find sources of diseases so growers can avoid or treat them to sustain their yields. But a bacterium called Xanthomonas can cause bacterial spot in tomatoes, severely reducing the annual tomato harvest for the growers who encounter it.

Most tomatoes come from transplant houses, which produce tomato seedlings to plant in the field, said Gary Vallad, a UF/IFAS associate professor of plant pathology and leader of a new study about tomato bacterial spot.

“Our findings, along with other published studies, show how field outbreaks of bacterial spot are linked to transplant production,” said Vallad, a faculty member at the UF/IFAS Gulf Coast Research and Education Center in Balm, Florida. “So, successful management begins at the transplant facility. Once the pathogen is introduced into the field, it is very difficult to manage, even with the best products on the market.”

The group’s findings should apply at any transplant production facility in the United States where bacterial spot is found, Vallad said. Study results also are likely relevant to other bacterial diseases such as bacterial speck and bacterial canker, he said. That’s because most transplant operations utilize overhead irrigation, which helps spread bacteria and is a major driver in bacterial disease outbreaks during transplant production, Vallad said.

A UF/IFAS-led team of scientists took Xanthomonas strains recovered from batches of seedlings produced at commercial transplant facilities and compared them to strains recovered later in the field season from the same batch of plants. Scientists found the bacterial strains in fields mirrored those found in transplant facilities.

Although diseased seedlings were removed at the transplant facility, the two Xanthomonas populations were the same, suggesting that strains found on the transplants play a large role as a source for field outbreaks of bacterial spot, Vallad said.

To come to their findings, researchers sequenced and compared genomes of bacterial strains collected from transplant and field outbreaks. They used small sequence changes scattered throughout the genome to differentiate strains. Those strains with fewer sequence differences were more closely related, Vallad said.

The new findings are critical as growers think about how to improve bacterial spot management.  The prevalence of copper tolerance in Xanthomonas limits the efficacy of fixed-copper compounds such as copper sulphate and copper hydroxide, which growers have long relied on to manage bacterial spot in transplant production and the field.

Other chemicals have shown limited ability to manage bacterial spot in the field. That’s especially true when environmental conditions — such as high temperatures, humidity and rain — spawn rapid disease development and spread, Vallad said. Improving bacterial spot management during transplant production, will not only improve transplant health, but limit the amount of Xanthomonas reaching the field where few effective management options are available.

The study is published in the journal Applied and Environmental Microbiology.

Brad Buck,  University of Florida


Gary Vallad inspecting tomatoes at the Gulf Coast REC in Balm Florida. Photo: Tyler Jones/UF/IAS

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