Spot on

Fungal leaf spot pathogens are some of the most common diseases threatening greenhouse crops. Yet while these blights are obvious once contracted, the variety of fungi limit growers’ abilities to accurately identify and thus treat outbreaks.

While most have minor effects on crops, the loss of aesthetic value among ornamental plants due to defoliation can make them worthless to growers who depend on luscious leaves and flashy florals for revenue. What’s more, less established crops may not have the hardiness to withstand extensive infection.

“The leaves are a power source for the plant in terms of photosynthesis,” said Aaron Palmateer, assistant professor and extension specialist in ornamental plant pathology at the University of Florida, IFAS (Institute of Food and Agricultural Sciences). “So often times when you have leaf spot problems, it’s going to affect the overall growth of the plant.”

How can growers prevent and treat these diseases to ultimately ensure healthy plant yields?

Proper ID
There are many fungi that cause leaf spots in plants. They generally produce spores that land on leaf surfaces and germinate when relative humidity and temperature become favorable, producing an infection in the leaf.

Equipping yourself with the knowledge of how to identify the various causal pathogens is the key to keeping your operation healthy.

“Preventive measures are by far and away the best way to manage these diseases,” Palmateer said. “If you’re growing something new for the first time, you need to do your homework to find out what types of problems and diseases you’re going to face.”

Some common strains among ornamentals are:

• Alternaria: Results in water-soaked spots that eventually brown and develop a yellow fringe.
• Cercospora: Results in light-green spots that turn gray or brown and develop dark margins, often dropping out to result in leaf holes.
• Downy mildew: Results in patchy lesions on leaf tops and grayish-white fungal growth on the underside of leaves corresponding with these lesions.
• Myrothecium: Results in dark brown to black spots that can initially appear water soaked, and may form a white fringe.
• Powdery mildew: Results in white spots that look like flour on top of leaves.
• Rust mildew: Results in small, orange, powdery spores that look like freckles across leaves.

Take preventive measures
Because leaf spot-causing fungi typically favor cool temperatures with high humidity and wetness, there are cultural measures you can take to make your greenhouse less hospitable to these diseases.

Avoid using overhead irrigation if possible to reduce moisture in your operation, favoring micro-jet or drip-type irrigation.

“If you can’t get away from using overhead irrigation, you want to water early in the morning so that that foliage has time to dry before evening,” Palmateer said.

Also ensure adequate air movement between plants to promote drying. Incorporate fans and avoid plant crowding.

“Everyone is trying to maximize use of their space, but with disease problems it can be an issue,” Palmateer said. “You want to adequately space plants to allow for good air movement.”

Overall sanitation is also key. Remove diseased material such as leaves and twigs as you identify infection, discarding entire plants if they become than 50 percent affected. Control weeds, as well, which often serve as pathogen carriers.

However preventive cultural methods aren’t adequate alone. Combine with a rotation of chemical fungicides and biological controls to keep plants disease-free.

“For (established) growers, you can make informed decisions,” Palmateer said. “If you have a particular problem, like if you know you’re going to have issues with Botrytis with your poinsettias, then get a feel for when that disease occurs in the growth cycle of the plant and then you can make preventative applications in that situation.”

Treatment after outbreak
Whether you’re taking preventative measures or treating an outbreak, chemical and biological measures are the most effective method for addressing fungal leaf spot. If you notice leaf spot spreading among your ornamentals, rotating chemical classes along with a round of a biological control is the most effective measure as the treatment variety will reduce the risk of resistance among the pathogen population.

“With these foliar fungi, resistance can build up very quickly because they just reproduce so fast.” Palmateer said.

Combining chemical fungicides with biological methods will also ensure effective treatment since the efficacy of biological measures can vary based on environmental factors. However not all products are compatible.

“It’s very important to make sure the product that you’re using with the biological is compatible in a rotation, because they’re living organisms – some chemicals may also reduce the efficacy of, or even destroy, them,” Palmateer said.

Whichever methods and products you choose to control foliar leaf spot, keep in mind that coverage is the most important factor.

“Some of the foliar fungi are sporulating on the underside of the leaves, so if you’re using contact fungicide and you’re applying but not getting the underside of the leaf, you’re not killing that fungus,” Palmateer said. “An airblast applicator can be very effective because you’re getting in the nooks and crannies of the plant.”

Jessica Hanna is a regular contributor to Greenhouse Management magazine.