A failure to treat potato seed for rhizoctonia is a false economy and risks yield losses of up to 30%, according to Scottish Agronomy’s Eric Anderson.

Rhizoctonia solani is a pathogenic seed and soil-borne fungus that affects a wide range of crops. In potatoes, it can cause patchy and uneven emergence via stolon and stem canker and leads to quality issues like black scurf and deformed tubers.

With farm input costs close-to-record levels, growers have been tempted to cut out rhizoctonia-active seed and soil treatments to save money.

Untreated disaster

Where growers took that risk last season the crop was, in some cases, an absolute disaster, warned Mr Anderson. “Inoculum in the seed caused reduced and uneven emergence, which in turn limited plant density with irregular potato growth. Ultimately, not treating went on to cost marketable yield.”

Seed harbouring rhizoctonia can also cause longer term issues by introducing the fungus into a field where it remains as soil borne inoculum, putting future crops at risk. Strategic use of chemical controls is critical, but there are some sensible practical considerations for its management first.

On lighter soils, soil-borne inoculum distribution is patchy and dependant on soil moisture and temperature. Crops grown in lighter soils are more likely to suffer higher losses to rhizoctonia.

More severe symptoms are frequently experienced in higher organic matter soils and there is a known link between stem canker and free-living nematodes, which can be managed.

“Some suppression of free-living nematode numbers is possible using a biofumigant like oil radish, which releases isothiocyanates (mustard gas) into the soil during growth,” Mr Anderson said.

Like for many other agronomic challenges, longer rotations will help reduce rhizoctonia inoculum levels in the soil and there is also evidence that green manures encourage microphagous (fungi-eating) nematodes that feed on the pathogen.

“Management of potato volunteers should be a priority, as the presence of groundkeepers can maintain inoculum levels in the soil.”

Soil moisture

Cold, dry soils hand the advantage to rhizoctonia and in essence, there is a race between the potato plant’s stem development and the hyphal threads of the pathogenic fungus, he explained. This means planting into warming, moist soils to speed up early growth is advised where possible.

Irrigation can also help, establishing stems much quicker and making them less susceptible to attack. Later in the season, the longer the interval between haulm destruction and crop lifting, the greater the incidence of black scurf.

Mr Anderson advised an ideal interval between desiccation and harvest should be less than five weeks.

Furthermore, trials had shown that plant densities also significantly affect rhizoctonia levels when crops are harvested later, so can help quantify risk.

“Using the same seed and variety, planting densities of 35,000 tubers/ha resulted in black scurf incidence rates of 9%. At 70,000 tubers/ha black scurf increased to 15% and at 140,000 tubers/ha to 34%,” noted Mr Anderson.

Chemical armoury

With such a wide host range, rhizoctonia risk-free situations are hard to find and means that fungicide treatments represent an important part of integrated control strategies.

For seed-borne rhizoctonia control there are currently four products approved, including RhiNo DSG (flutolanil) powder applied at 2kg/t on the planter. RhiNo (flutolanil) and Maxim FS (fludioxonil) are liquid seed tuber treatments applied at 200ml/t and 250ml/t, respectively, on a specialist applicator over a roller table.

Trials in Scotland have shown that liquids and powders are equally effective against seed borne rhizoctonia. However, in Scottish Agronomy’s trials, the active substance flutolanil – contained in RhiNo DSG powder and RhiNo liquid – had given better control against soil-borne rhizoctonia than fludioxonil, as used in Maxim 100FS.

Mr Anderson noted that while most growers had switched to using a liquid for ease of use, the powder-based control option still has a role to play, particularly in 2023.

Reduced dormancy

Last year’s hot, dry summer reduced the innate dormancy of daughter tubers and some seed crops sprouted in the ridge before lifting. Early dormancy break has commonly been recorded in seed stores this season.

“This is significant because the liquid product labels prohibit use on sprouted seed or where sprouts are being damaged during the treatment process.

“Given that availability of Honesty (fluxapyroxad) will be limited in 2023, RhiNo DSG is the only practical remaining choice. It has no label declaration on sprouted seed, so could still be applied on the planter,” he pointed out.

Whichever seed treatment product is chosen, Mr Anderson warned against reducing application rates below label recommendations. It may seem like a good way to cut costs, but protection levels reduce sharply below recommended rates, so it is a false economy, he argued.

“Any decision should also consider the effectiveness of each choice against the disease spectrum known to be of concern for each crop,” he concludes.

Don’t forget soil-borne inoculum

Soil-borne rhizoctonia cannot be adequately controlled by seed treatments alone, so in some circumstances Mr Anderson said that combining seed treatment with an in-furrow spray would be required to extend the spectrum of activity.

The agronomy group’s trials had shown that an application 3l/ha of azoxystrobin provided the most consistent protection against soil-borne rhizoctonia.

But, he said that active substance selection should be based on the crop purpose. Azoxystrobin can occasionally check crop emergence and while main crop potatoes can grow through that check, yields could be affected more in salad and seed potatoes.

“Because there is no emergence check associated with Allstar (fluxapyroxad), it may be a better choice for crops grown for these markets,” he said.