by Tamara Scully
Cool climate grapes are prone to sour rot. Infected grapes not only smell like vinegar, they are discolored with tan or red hue to infected berries, which eventually become watery. Sour rot doesn’t have a single cause or vector. Many organisms, including fungi, bacteria and insects play a role in the disease. But one thing is needed for sour rot to develop: a wound to the grape.
There are many reasons that grapes may become wounded. Those varieties that grow in tight clusters are more prone to wounding, possibly from berries rubbing against one another. Wet conditions which cause berry splitting are often factors in sour rot development. Sour rot is more prevalent under warm weather conditions, in the upper 60s, than when cooler temperatures prevail. Canopy management practices can also play a role in susceptibility, and enhancing airflow and sunlight exposure can help to decrease sour rot development. Berries with sour rot ooze, and the disease is rapidly spread to nearby fruits. Fungicides are generally not effective at controlling or preventing sour rot.
Grapes are most prone to sour rot close to harvest, when Brix levels are at 15 or above. Disease can be initiated when rain occurs anytime after this level of maturity is reached. Questions surrounding the role of microorganisms, fruit flies and the epidemiology of sour rot have been the focus of recent research.
Grape varieties that grow in tight clusters, rubbing together, are more likely to cause skin wounding than varieties which grow in looser configurations, and are more prone to sour rot. As wounded grapes leak juice, ethanol is created by yeasts present on the grapes. Bacteria then transform the ethanol into acetic acid.
“A distinguishing feature” of sour rot is the vinegar smell, Megan Hall, Ph.D student in Plant Pathology and Plant Microbe Biology, New York State Agricultural Experiment Station said. “Vinegar doesn’t spontaneously develop. There is a precursor.”
Hall has been conducting research into the etiology of sour rot during the past few years. To begin to understand the causative agents, Hall washed grape clusters to isolate the microbiome, and found 1,300 species of microorganisms. The four most prevalent were inoculated into wounded grape clusters in the laboratory, in various combinations. Out of 10 combinations inoculated, eight of them did not cause disease unless drosophila — fruit flies, either common or spotted-wing — were present.
“It requires a wound site,” Hall said of sour rot.
Fruit flies, although also attracted to acetic acid, appear to play an important role in the development of acetic acid and sour rot symptoms. Without the flies, the ethanol isn’t readily converted to acetic acid. The presence of flies was seen to correspond with a rapid increase in acetic acid accumulation.
Trial studies in 2013-2015 used antimicrobial and insecticides together in order to gain control of sour rot symptoms in infected fields. Chemical protection was required before grapes were symptomatic in order to be effective. To gain control of sour rot symptoms, Hall found that both an antimicrobial and an insecticide were needed, and that they needed to be applied pre-symptom appearance.
“You can’t reverse sour rot symptoms, but a spray application may have helped it from developing as quickly,” Hall said of trial results.
Trial data shows that Vignoles vines in the Finger Lakes region treated with both antimicrobial and insecticidal sprays post-veraison, on a weekly basis, before any sour rot symptoms appeared, had significant reduction in sour rot symptoms than the untreated vines did.
Canopy Management
Two different types of grape vine training systems were studied to assess whether management systems can play a role in the development of sour rot. In Vignoles clusters, vines under the high wire cordon or the flat cane vertical shoot position (VSP) training systems were studied.
The high wire cordon system was found to have “significantly more disease” pre-harvest, Hall said.
It is hypothesized that the VSP system, the upward positioning of the shoots, allows increased airflow through the vines. This creates a less than ideal environment for bacterial pathogens, and disperses fruit flies. Under an “umbrella” type of canopy created with high cordon wires, airflow is stagnant, and flies and pathogens can thrive.
Leaf removal is one possible way of reducing berry cluster compaction. Fruiting zone sprays can also be used in an effort to loosen clusters. Looser berry clusters are less susceptible to wounding injuries.
Environmental factors
Hall has also conducted research in Tasmania. In two orchards, one with symptoms of sour rot and one where the presence of sour rot was a possibility, fields were monitored for dew point temperatures every 10 minutes. When the maximum daily dewpoint was dropping, sour rot did not develop. Under conditions with an increasing maximum daily dewpoint, sour rot symptom severity was compounded, demonstrating a correlation.
Sour rot is “relevant to many people beyond our community right her,” Hall said, noting that sour rot is a worldwide disease issue, causing economic loss to the industry.
Sour rot requires conducive conditions to develop. Fruit flies, wounded berries, warm and wet weather and the presence of bacteria and yeast all seem to be required for the disease to occur. Researchers aren’t yet sure of the specifics of transmission and control, however growers can take precautions before disease symptoms occur. Once symptoms appear, control is difficult.