Top Crop Manager

Features Fungicides Seed & Chemical
Checking up on Rhizoctonia and early blight

Rhizoctonia is a fungal pathogen found wherever potatoes are grown, and one to watch because it can survive in the soil for many years. “It can gain a foothold at the start of the season, and can certainly severely hinder stand establishment,” says Dr. Rick Peters, potato pathologist at the Agriculture and Agri-Food Canada Crops and Livestock Research Centre in Charlottetown, Prince Edward Island. “It’s especially important to control it in cooler, wetter years. When succulent underground plant tissues are moist, they are ripe for attack.”

Peters was called out to several fields in early summer 2010 to do disease diagnosis, and found that Rhizoctonia (or “rhizoc” for short) was the culprit. “The stands were poor in vigour and in health,” he says. The plant damage caused by rhizoc is apparent early in the season, and later in moist soil, the black scurf structures (sclerotia) form a hard crust on the tubers as an overwintering stage. If there are any present at all on seed in the spring, the disease can spread. “The number is two percent,” stresses Peters. “If there’s a higher percentage of the surface area of the seed covered with sclerotia than that, you’re going to be in trouble. After your plants have come up, you don’t have any recourse.”

Prevention the key
Once the seed is cleaned, Peters advises using seed treatments. However, he says that while seed treatments will help reduce some infections, they are not a substitute for clean seed. “In addition, if you control the pathogen on the seed and not in the soil, or vice versa, you are in trouble,” he says. “You must do both.” Therefore, in addition to seed treatments, also use a crop protection product in furrow at planting.

Length of crop rotation is also very important. “Our research has shown that three years is better than two years, and four years is better than three years,” says Peters. “The fungal soil population just breaks down over time, so the longer it’s left without a host, the better.”

Canola and some of the mustards (for example, Brassica sp.) have shown particular rhizoc suppressant abilities in Peters’ research studies. “You can grow canola and still get some suppression of rhizoc in potatoes the next year because it excretes some chemicals from its roots that suppress the fungus,” says Peters. “However, you will get a much greater effect if you till the canola into the soil as a green manure. The canola tissues will release toxins that kill the pathogen in greater amounts this way, and tilling it in will also increase microbial activity in the soil, which also in turn inhibits rhizoctonia growth.”

Lastly, if a grower is not sure which pathogen he or she is dealing with, it is best to get infected potato plant tissue samples to the nearest government diagnostic lab as soon as possible, advises Marleen Clark, a plant disease diagnostician with PEI’s Department of Agriculture in Kensington. “An accurate disease diagnosis can then be correctly treated,” she says, “and in the end, an effective treatment can save farmers money.” 

Early blight fungicide resistance
Alternaria early blight is another fungus to watch, particularly in the Prairie provinces. It is really only an issue in the Maritimes when drought conditions appear; very dry weather weakens and stresses potato plants, putting them at risk for infection.

Early blight is also a concern because in a 2008 study, Peters found that the pathogen demonstrated widespread resistance to strobilurin fungicides (which include Group 11 products) in the potato fields of Ontario, Manitoba and Alberta. “None was apparent at that time in PEI, but continued monitoring would be advisable,” says Peters.

Similar to many other fungal pathogens, early blight can survive between potato crops in residue, in soil, in infested tubers and on other hosts. To manage early blight, it is important to use clean seed, destroy diseased vines (burning is recommended), maintain soil fertility and use as long a crop rotation as possible.

Early blight first becomes visible as very small brown spots on older leaves. These spots grow into concentric rings (resembling a “bulls-eye” target), which results in extensive leaf loss and lower yields. Infection on tubers appears as dark, sunken, roundish areas with raised borders.

Research update: DuPont Vertisan fungicide
A new fungicide for treating and preventing Rhizoctonia, early blight, powdery mildew and grey mould, has been submitted for registration to the Pest Management Regulatory Agency. Research indicates that DuPont Vertisan offers control of these foliar and soil-borne fungal diseases in broad-acre crops, including canola, pulses, potatoes, sunflower and sugar beets.

The new active ingredient in Vertisan is called penthiopyrad. It stops the growth of plant pathogenic fungi by blocking cell respiration, and inhibits spore germination and mycelial growth. “Vertisan will be a valuable resistance management tool because of its Group 7 chemistry and next-generation Complex II mode of action,” says Jim Irish, specialty products manager at DuPont.

Vertisan moves through treated leaves to stop infection and protect the entire leaf with preventive and, on certain diseases, curative activity. It offers strong residual control, excellent crop safety, outstanding rainfastness and tank-mix compatibility.

A submission has been made for Vertisan to be applied by ground and air. For Rhizoctonia, it will be applied in-furrow at planting.