Dealing With Diseases

In many ways, diseases are more difficult to control than insect pests. The organisms are microscopic and you often don’t see the damage until it’s too late. Spores reproduce rapidly and infections can take hold in less than 10 hours. Irrigation systems can spread pathogens throughout your facility.

While some diseases are water borne, for others water is just a mode of transportation to spread the disease. The most common water molds are Phytophthora and Pythium. Pathologist Mary Hausbeck of Michigan State University explains these organisms are more like brown algae than fungi. “They are a diverse group, adapted to an aquatic environment. They cause damping off, root rot and crown rot,” she adds.

The goal is not to keep water completely sterile but to know how much of a pathogen’s presence is economically significant. “Take a sample. Learn what’s there. You can also use susceptible plant tissues, like pears and cucumbers, as bait,” she says.

Keeping Pathogens At Bay

Monitoring pathogen levels is especially important for growers who recycle and recirculate their irrigation water. Paul Fisher of the University of Florida presented an installation and operating cost comparison of water treatment technology options. For a complete look at these, visit

He and his colleagues and students have surveyed 25 growing operations in the United States, assessing water sources and establishing recommended levels for organic loads – bacteria, years, molds and solids. Petri film cards are a quick way to assess but further analysis is required to determine which pathogens are detected. The film cards are like sticky cards and work like a petri dish culture.

Most of the investment in systems is plumbing and installation. For one grower Fisher worked with, a water treatment system paid for itself within the first year by eliminating crop losses. “You need to prioritize water quality and risks,” he explains. “You may want to use non-recycled water on young plants, geraniums and crops susceptible to sudden oak death (Phytophthora ramorum). Keep it simple. Select technologies that suit your situation.”

In many ways, it is like becoming a municipal water manager, he says. Keeping chlorine levels to 1-2 ppm is especially important because phytotoxicity occurs in plants at levels of 4 ppm and higher.

Persnickety Plant Diseases

Mildews continue to be an issue and are showing up on more crops, says plant pathologist Margery Daughtrey of Cornell University’s Long Island research station. The strain of powdery mildew on roses attacks the genuses of rosa and prunus. The strain that attacks verbena also infects cucurbit crops. Verbenas vary in susceptibility by cultivar and she recommends choosing resistant varieties. Many chemical controls are available for powdery mildew, including Strike, Terraguard, Banner Maxx, Eagle, Compass O, Insignia, Camelot, Phyton 27 and Decree.

Downy mildew has been showing up in coleus the past few years, she adds. While downy mildew is a water mold, it affects plant foliage instead of roots. Coleus symptoms include leaf spots, curling and mildew underneath. This type of mildew will affect the mint family, inc

luding basil. She recommends moving away from susceptible plants but most coleus varieties are susceptible.

Bacterial diseases are still a prime issue for geranium crops. Xanthomonas was first discovered in geraniums in 1888 and more recently a strain of Ralstonia brought global geranium producers together to develop protocols for clean stock and culture indexing. Daughtery recommends growers use copper-based products to sanitize their greenhouses and keep seed-grown geraniums separate from vegetatively propagated ones.

Diagnosing Disorders

How do you know if the symptom you’re seeing truly is a disease? Kim Williams from Kansas State University presented an overview of nonpathogenic physiological disorders that are commonly mistaken for diseases. These can be triggered by nutrient deficiencies, temperature extremes, too much light or poor moisture management. Sometimes plants will lose water faster than they are transpiring.

Heat delay is a common problem producing mums in the summer. Symptoms include bud malformation and deformed florets. Williams recommends keeping night temperatures lower than 82 degrees the first two weeks of the crop, opening black cloth at night to vent and choosing heat-tolerant cultivars.

Alternatives To Fungicides

Plant Pathologist Dave Norman of the University of Florida presented an update on biologically derived disease control products. While many have been worthwhile as preventive products, once plants are infected, conventional chemical products are still the most effective course of action, he says.

“Biopesticides are derived from natural materials,” Norman explains. “There are 195 active ingredients and 780 products available. They fall into two major classes. Biochemical products are naturally occurring with nontoxic mechanisms. Microbial pesticides produce micro organisms to combat the disease. Right now, I don’t have any impressive fungal control data, but I do have non control and slight control data.”

Another new group of products is systemic acquired resistance – stimulating a plant’s immune system by triggering a response to protect the plant. Actigard, a new reduced-risk product from Syngenta, is one of these. “It has no direct activity on pathogens, so there are no resistance issues,” Norman says. “The chemical makes the plant think its under attack. The response lasts eight to 14 days. But as with most biopesticides, if there already is a disease outbreak, it’s too late to treat the plant. These products work best for prevention and rotating with other chemical controls to minimize resistance.”

Other new products in the works are Citrex, a vitamin C-based product coming through the IR-4 channels. SePro and North Carolina State University also are working with a product derived from sea sponges with antifungal, antibacterial and antiviral properties.

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