Fusarium wilt, caused by the pathogen Fusarium oxysporum f. sp. niveum, is a major concern for watermelon growers in the U.S., and is even more severe for those producing the Southeast.
Bhabesh Dutta, Assistant Professor of Extension Vegetable Pathology at the University of Georgia, has teamed with other researchers from the university in an effort to discover the role that soil fertility plays in the severity of Fusarium wilt in watermelon.
While the research has not yet concluded, Dutta and his colleagues have uncovered preliminary data on other crops suggesting micronutrients in the soil do play a role in disease severity.
The project was proposed in November 2015 to the National Watermelon Association, and was funded the following month, Dutta says. Research began March 2016 and is still ongoing
“What we have seen with other diseases is that soil fertility and the concentration of micronutrients in the soil did affect certain diseases,” he explains. “We have tried this concept on peppers for the last four years and discovered it holds true for bacterial spot disease.”
Fusarium wilt is a soilborne pathogen, which is why the effect of soil fertility is being studied on disease severity, he explains.
Thus far, the team has conducted preliminary trials where it compared watermelon plants with severe symptoms and plants showing lesser disease symptoms.
“We took soil samples as well as tissue samples from plots displaying a difference in disease severity, and analyzed all of the micronutrients for both samples. After that, we did regression analysis of individual micronutrient concentrations and their ratio to one another with the disease-severity ratings,” Dutta says.
At this stage of the research, we are trying to study the proof of concept with one watermelon variety and one soil type, with the intention of extending and trying out this concept on different soil types as well as different varieties.
“I hope to see differences,” Dutta says. “The genotype of the host determines the resistance of the pathogen. Certain watermelon cultivars have resistance to one or few races of this pathogen; and depending on that interaction, we’ll see different disease reaction or symptoms.”
The Fertility And Disease Relationship
When researching the effects soil fertility had on peppers and bacterial spot, Dutta and his colleagues noticed there are certain nutrients in the soil and in the plant tissue that can affect the systemic acquired resistance in plants.
“They observed a strong relationship between bacterial spot severity in pepper and concentrations of key cations. These cations also act as cofactors for superoxide dismutase (SOD) enzymes that detoxify reactive oxygen species produced in plants upon pathogen attack,” Dutta explains. “As a result, hydrogen peroxide is formed, which acts as precursor for salicylic acid (SA) formation. SA has been proposed as the signal molecule to initiate the systemic acquired resistance (SAR) pathway. We hypothesize that cation concentrations in pepper plants can modulate important enzymes in the pathway of SAR against pathogen attack. This is what we think is being affected by the micronutrients in the soil.”
He also says the team looked at two resistant genes in the SAR pathway of the plant. “We also think that these minerals in the soil may affect the resistance, thereby affecting the severity of the symptoms. We are in the process of analyzing the samples, and we will soon submit the report to the National Watermelon Association. At this point, however, I’m not able to reveal a lot,” he adds.
Disease Onset And Symptoms
Fusarium wilt is a root pathogen that causes pre- and postemergence damping off of young seedlings and wilting of mature plants. The pathogen blocks the vascular tissue of the plant, resulting in a disruption of water movement from the soil to the upper canopy.
“The first thing that you’ll see is that one vine will die, followed by several more. Then you will start seeing gradual yellowing of the leaves. As the disease progresses under certain conditions, the entire plant will wilt and die,” Dutta says.
If the infection is moderate, the plants may still flower, but the quality and yield of the plant may be severely affected.
“In some cases, you may see fruiting, but people have seen under several conditions that the disease severity can be as high as 80% and more.”
According to Dutta, research findings from other scientists suggest that disease severity can be greater at soil temperatures between 25°C to 27°C, and is greatly reduced above 27°C. Dry conditions also can aggravate the problem.
“If the vascular system is already compromised with the pathogen, and cool and dry conditions prevail for certain duration, the plant gets stressed even further. The stressed plant may increase its water uptake, but it can’t because of the clogging of xylem vessels and hence, it wilts,” he explains.
Additional Management Tactics
The best method to manage Fusarium wilt in watermelon is to use a combination of chemical, biological, and cultural controls. The strategies include the use of disease-free transplants and seed, resistant cultivars, crop rotation, and timely application of fungicides, Dutta says.
“If you know you have a Fusarium wilt problem, avoid planting in fields with known Fusarium history as this pathogen can survive for a long period in soil,” he explains. “A five- to seven-year crop rotation is recommended with other non-watermelon plants.”
You can still plant other cucurbit crops because Fusarium wilt is highly specific and will generally not cause infection on other cucurbits, Dutta adds. He also highlights the importance of buying disease-free certified seed from a reputable source.
Next Phase Of Research
One of Dutta’s colleagues, Dr. Pingsheng Ji who received backing from a grant from USDA, will be focusing on the diversity of Fusarium wilt races in the southeastern U.S., the effect that different cover crops will have on the population of the pathogen in the soil, and will be assessing the mode of Fusarium oxysporum f. sp. niveum seed infection in watermelon.