By Melissa Piper Nelson
In a growing season complicated by wild weather laced with torrential rain, hail and heavy winds, the watchword continues to be hyper-vigilance when scouting for horticultural disease symptoms, advised Dr. Carla Burkle, Penn State vegetable production specialist and horticulture team member.
“Things can and will happen quickly and you have to be prepared to act,” she said. Burkle, who strongly encourages the preventative triad of clean production and harvesting techniques, rotating crops and growing resistant varieties for a healthy cropping season, is concerned with the amount of downy mildew now showing up on cucumbers and butternut squash. Spotted in at least four south central Pennsylvania counties, Burkle cautioned that this group defined as “highly specialized obligate parasites” may continue to attack a host of vascular plants across many regions.
“Downy mildew,” said Burkle, refers to a disease symptom caused by several different species of oomycetes in the order Peronosporales that affect many vegetable and ornamental plants – it is not a single pathogen or disease. Downy mildew usually appears on the underside of leaves and can be confused with gray mold. When leaves are destroyed, they cause the plant to lose photosynthetic area with subsequent die-off. Speaking of cuburbit plants (a plant of the gourd family Cucurbitaceae), which includes melon, pumpkin, squash and cucumber, as leaves die the plant is not able to support fruit production and the fruit will be small or misshapen. The pathogen doesn’t directly affect the fruit, according to Burkle. Grapes also can be distressed from the pathogen, although through a different species of an oomycete pathogen, Plasmopara viticola.
“Downy mildew is a problem that shows up each year, working its way from the southern states to the north,” said Burkle. “We are always cautioning growers to be on the lookout for the symptoms and to treat with fungicides if necessary. Organic growers need to determine the best management practices within their plan too,” she added. “Be on guard and tracking for problems.”
Downy mildew is attracted to high humidity levels at the leaf surface and as noted in grower bulletins, “the time from infection until new spores form can be as short as four days” or up to 10 days. “Losses can definitely be severe if the pathogen is present and preventive and curative measures, including effective fungicide application, aren’t used,” cautioned Burkle. “A grower may not lose the entire crop but major losses are possible.”
Referring to seasonal weather complications, Burkle noted, “Crops that are covered in ponding water are stressed and this can make them more susceptible to pathogen invasion. Hail can create physical wounds that allow pathogen entry. The high humidity we’ve experienced is favorable for many pathogens to initiate infection. Wind-driven rain can spread fungal and bacterial inoculum and create wounds in plant tissue where the disease inoculum may enter.”
Prevention taken throughout production and harvesting may offer at least a barrier to disease problems, Burkle said. “Clean harvesting and rotation are cultural techniques that can reduce the potential for pathogen inoculum to build up in a field. By doing this, the grower may not have to contend with disease inoculum being present from day one, and can buy some time until it enters the field from other sources, such as from spores blown in on the wind or carried in on affected plant material or contaminated tools.”
Using resistant varieties can be helpful with diseases that are still difficult to manage even with a good spray program, according to Burkle. “For various reasons, some pathogens are very effective at causing disease, and growing a resistant variety is another tool that can help the grower manage the disease if it’s a consistent problem. Using a resistant variety doesn’t negate the need for ecological disease management, and chemical management may still be necessary if disease pressure is high or the variety’s resistance is not robust.”
During this growing season that, in many areas, has seen almost unprecedented moisture and flooding, growers are being urged to keep a close eye on field conditions that could impose serious crop losses.