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Breaking the ascochyta disease cycle

Farmers who grow pulse crops know that ascocyta can be a serious problem. Break the cycle on your farm

Ascochyta blight is a seed-borne disease that can cause yield loss in peas, lentils and chickpeas.

Fortunately, each strain of ascochyta is crop specific — the strain that infects your lentil crop will not impact your chickpeas. This makes crop rotation the first line of defence against ascochyta.

“It’s important to start with a good crop rotation so that we are not increasing the amount of disease that we are getting,” says Faye Dokken-Bouchard, provincial plant disease specialist with the Saskatchewan Ministry of Agriculture. “Rotation is key because the ascochyta pathogen can only survive on the residue of the specific crop that it infects, so if you give that crop residue time to break down in the soil, the disease will break down with the residue as well.”

The primary source of infection for ascochyta is crop residue, so by not planting peas after peas or lentils after lentils, the specific ascochyta strain that may be present in the previous year’s residue cannot infect the current crop.

Not only does crop rotation help break the disease cycle but, as part of a wider integrated management strategy, it can also help maintain resistance to ascochyta that has been bred into pulse crops for the long term.

“If we continue to grow the same crop with some resistance to a disease there will eventually be a low level of the pathogen that is able to overcome that resistance and will build up,” says Dokken-Bouchard. “Using an integrated pest management strategy overall is a good way to deal with all diseases and ensure that resistance remains strong, because you are using more than just the genetics to deal with the pathogen. If you are also using other tactics, like crop rotation and resistant varieties in combination with fungicides, you have a much better chance of those products and varieties lasting and continuing to be useful and available for a long time.”

Controlling ascochyta

Farmers who find ascochyta in their crops need to take into account the amount of disease present, the weather outlook (particularly as the crop approaches the flowering stage) and the expected yield potential when they decide whether or not to spray to protect the crop.

The number one thing is to protect the flowers, says Dokken-Bouchard. “It’s important to protect the flowers and get the fungicide in there before the canopy closes, when you won’t be able to get as much of the product down into the lower parts of the crop,” she says. “The window of opportunity to spray depends on when and how long the crop is flowering for. If the crop continues to flower later in the season, you still might to continue to have a window to apply fungicides, but you will have to make sure that there is enough time for those flowers to actually produce a yield. It takes about a month to go from flower to seed, so you don’t want to wait until too late in the season either.”

Ascochyta blight is favoured by wet weather, particularly frequent showers. The optimal temperature for infection and lesion development is around 20 C. If the canopy remains dense and wet into the flowering stage, lesions will continue to develop on lower leaves and stems.

Scouting for Ascochyta

Scouting for ascochyta should begin from the time seedlings first emerge to just before flowering to help farmers decide if they need to consider an early fungicide application.

“Ascochyta has multiple disease cycles, so if you see early symptoms then you know that the disease can spread if there is rain splash or good conditions for disease,” says Dokken-Bouchard. “The biggest yield impact results from the disease infecting the plant when it is flowering. So that’s the best time to be scouting and spraying.”

Consulting field histories, and not just relying on data from annual disease surveys is also important says Dokken-Bouchard. “Scouting now is important but it’s also about knowing what trends there have been in their own fields and what diseases they have had in the past, which will help them better prepare to target diseases that may show up,” she says.

Using a fungicide

If symptoms do not move beyond the lower third of the plant canopy at the flowering stage, risk of yield loss is low and fungicide should not be applied.

Fungicide control may be warranted if:

  • at least 40 per cent of the bottom third of the crop canopy is showing symptoms and symptoms are progressing into the middle third of the canopy; and,
  • the weather has been humid and rain is in the forecast; and,
  • crop yield is expected to be high enough to justify the cost.

Pea varieties rated as having “fair” resistance to ascochyta blight rarely benefit from a fungicide application.

Regardless of yield benefits, seed growers may want to make a fungicide application to protect the quality of their harvested seed.

Ascochyta in field peas

Although infection can occur at any time during the season, and early symptoms are often detected on emerging seedlings, it is infection at the flowering stage which causes greatest yield loss in field peas.

Yield losses of five to 15 per cent are common in regions where the disease is established and wet conditions occur during pod development. If flowers and pods become severely infected both yield and seed weight are likely to be reduced. The stem base can also become infected causing foot rot, which can lead to lodging, making harvest more difficult.

Three species of ascochyta can infect peas; Mycosphaerella blight, Ascochyta pisi and Phoma medicaginis (foot rot). Mycosphaerella blight is the most common species found in pea crops across the Prairies, although Ascochyta pisi is becoming more prevalent in the south-western corner of Saskatchewan. Mycosphaerella blight produces ascospores on crop residue that are released over the spring and summer and can be blown by wind over several kilometres to infect other pea crops, which can be a limitation to the effectiveness of crop rotation in dealing with this particular strain. Other strains of ascochyta release spores which remain more localized and are transported via rain splash to adjacent plants.

Choosing seed

Because ascochyta is seed-borne, using the right seed can help limit damage from ascochyta.

Choose a variety with some resistance to ascochyta. Some varieties of peas are rated as having “fair” resistance to the disease and some lentil varieties are listed as having “good” resistance. It’s also helpful to choose varieties with good lodging ratings and avoid fields that have received excess nitrogen fertilization — lodging can increase disease susceptibly.

Ideally, it’s best to plant seed with less than 10 per cent ascochyta infection. Seed with infection levels higher than 10 per cent should be treated with a fungicide seed treatment. †

Peas

Mycosphaerella blight and foot rot: Look for early symptoms as small, purplish brown irregular flecks on lower leaves, stems, and tendrils. Flecks enlarge and coalesce so that lower leaves become completely blighted. Small, dark, pepper-like dots called pycnidia may also be visible within the lesion. Severe infections may lead to foot rot, with purplish-black lesions at and above the soil line, which often causes lodging. Pod lesions are initially small and dark and may progress to give small, shrunken or discoloured seed.

Ascochyta pisi: Look for tan or brown lesions with a distinct dark brown margin. Small, dark, pepper-like pycnidia are usually visible within the lesion.

Lentils

Check leaves, pods and stems for white to tan spots with a darker outside margin, which may contain black, pepper-like dots. Infected seed can be a brownish colour.

Chickpeas

Check all above ground portions of the plant for dark, sunken lesions which contain rings on the outer margin. Spores (small, pepper-like dark dots) may be present in the lesions. †

About the author

Contributor

Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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