There are only a few diseases that significantly affect chickpeas. Ascochyta blight is a foliar disease that can impact a chickpea crop with up to 90 per cent yield loss in Kabuli and 50 per cent yield loss in Desi chickpeas. Ascochyta blight can also affect seed quality resulting in lower grades. In chickpeas, Ascochyta blight is much more aggressive than in lentils or peas, and is caused by a different Ascochyta species.
The pathogen overwinters on chickpea residue and seed. Both asexual spores and sexual spores can be produced on the residue. The sexual stage produces ascospores, which can spread several miles in the wind and are believed to be responsible for early season lesions. These ascospores are produced by genetic recombination, meaning the population can become genetically diverse. Research carried out at Agriculture and Agri-Food Canada, Saskatoon, has identified 15 races of Ascochyta rabiei in Western Canada. This not only makes breeding for resistance more of a challenge, it increases the likelihood of fungicide-resistant strains developing.
Symptoms of Ascochyta blight include tan or brown lesions on stems, leaves, and pods. The lesions may girdle entire stems, causing them to wilt and die. Dark fruiting bodies, called pycnidia, are formed in the lesions. The pycnidia ooze spores in wet and humid conditions. Spores are spread by rain, thus infection is aided by weather with frequent showers. Plants will show lesions approximately four to seven days following infection. If the weather turns warm and dry, infected plants may survive, but will be delayed in maturity and produce lower yields. Ascochyta blight is also seed-borne, so the use of disease-free seed is critical. Ascochyta blight is also capable of surviving for several years on crop residues in the soil. A minimum four-year crop rotation will reduce the risk of infection.
Fungicides registered for control of Ascochyta blight in chickpeas work in two ways: protective and curative. Protective fungicides work to control disease by creating a barrier on the plant’s surface to prevent the spores from germinating and infecting the tissue, and are most effective if they are applied preventatively, or before the fungus penetrates the plant. Curative fungicide activity is limited to the early part of the infection, and only for the first 24-36 hours after spore germination and infection. These curative products do not repair tissue that has already been damaged or killed by the fungus. No fungicide will protect against disease already established in the plant or once lesions form. All fungicides will protect against spores entering the plant (protectant activity), while only some products have curative activity. Fungicides that are currently available will provide protection against disease for no longer than two weeks. Refer to individual labels for more information.
The following are some key guidelines to a good integrated pest management program for Ascochyta in chickpeas:
- Rotation: Three- to four-year crop rotation to non-host crops is necessary to reduce spores from previous crop stubble.
- Field Choice: Avoid planting chickpeas adjacent to previous year’s chickpea fields to reduce spread of residue and wind-borne spores and use non-host strips at field edges. Field selection should be at least 500 metres away from fields that had a chickpea crop the previous season.
- Type of Chickpea: Choose class based on risk of Ascochyta. Kabuli chickpeas are much more susceptible to Ascochyta blight compared to Desi chickpeas. Unifoliate Kabuli varieties appear to be much more susceptible to severe Ascochyta blight, compared to fern leaf Kabuli.
- Variety Choice: Choose varieties that are as disease resistant as possible. Currently the best resistance level available is fair, and therefore, select varieties with a “fair” rating for Ascochyta, instead of “poor” or “very poor”.
- Seed Quality: Plant seed that has zero levels of Ascochyta, or levels below 0.3 per cent Ascochyta rabei. Use seed treatments if needed.
- Seeding Rate: Target optimum plant densities (Kabuli 38-44 plants/m2, 3-3.3 plants/ft2, Desi 44-50 plants/m2 or 3.3-4.5 plants/ft2). Increasing plant density above the recommended range with highly susceptible varieties may increase disease severity.
- Fungicide Use: First application at early seedling stage (use earliest label staging) is critical to prevent or slow early development. Follow-up field scouting and additional fungicide applications may be necessary.
- Scout Often: Begin at the seedling stage, two to three weeks after seeding. Scout every three to seven days during the seedling stage. Rain and/or high humidity means scouting frequency should be increased. If conditions are drier and the chickpea plant gets past the seedling stage, scouting frequency can be decreased to every seven to 10 days.
A disease decision support checklist for Ascochyta has been developed to aid in determining risk rating to Ascochyta, and to determine appropriate management strategies.
Management of Fungicide Resistance: This is extremely important for Ascochyta in chickpeas due to the genetic diversity of the pathogen, and the fact that isolates with resistance to strobilurin fungicides have been confirmed in Saskatchewan. If a pathogen is resistant to one fungicide in the strobilurin group, it will be resistant to other fungicides in that group.
The following guidelines, adopted from the North American Fungicide Resistance Action Committee, are recommended to prevent the increase of fungicide-resistant fungi:
- Do not use a fungicide that contains only a strobilurin active unless it is tank mixed with a non-strobilurin fungicide.
- Rotate the use of a fungicide with a strobilurin product in the mix (or tank mixed) with a non- strobilurin product.
- Do not use more than two applications per year of any fungicide containing a strobilurin on the same field.
- Do not apply more than two applications of the same group in a single growing season (except for chlorothalonil, which can be applied three times).
Seed Rot, Seedling Blight, and Root Rot of Chickpeas: These are caused by a complex of pathogens including species of Pythium, Fusarium, Rhizoctonia and Botrytis. These pathogens are present in all Saskatchewan agricultural soils and can infect and kill individual seedlings from germination to the early flowering stage. Seed rots and seedling blights are most severe when the soil is cool or saturated, and seedling emergence is delayed. Infected seed may fail to germinate. Infected seedlings will usually turn yellow, wilt and then die. Stems may be girdled and discoloured at or just below the soil surface and roots may be rotten, allowing the plants to be pulled easily from the soil. Kabuli chickpeas are especially susceptible to rots due to its thin, zero-tannin seed coat. Research has shown chickpeas to be partially resistant to Aphanomyces. If Aphanomyces has been confirmed in a field, chickpeas are a pulse option that can be used in rotation instead of peas or lentils, which are susceptible hosts.
Botrytis Grey Mold: Botrytis attacks chickpeas, both at the seedling stage, and in advanced stages. Botrytis grey mold of seedlings may spread down a seed row, resulting in a series of yellow or dead seedlings. Botrytis grey mold is also favoured later in the growing season by dense canopies and moist conditions. Botrytis is usually most evident after flowering and is common on pods, resulting in shrunken, discoloured seed. The infected area is often covered by a dark grey, fuzzy, fungal growth. Botrytis pathogens can survive in the absence of host crops so rotation has limited effect on disease level. Fungicides are available for the control of late season development of Botrytis grey mold on chickpeas, but needs to be applied prior to symptoms showing to be effective.
Sclerotinia White Mold: This disease attacks chickpeas grown in conditions of high rainfall, which produces dense crop canopies. Sclerotinia white mold is more common in crop rotations that include other susceptible broadleaf crops such as canola, mustard, lentils, or peas. Symptoms usually occur in patches, typically in heavier crop areas. Infected plants are initially paler green and the diseased tissue may be covered by a white, cottony, fungal growth. The plant later becomes bleached in colour and the infected area will easily shred apart, revealing small black fungal resting structures called sclerotia bodies. Sclerotinia becomes evident later in the growing season and if found, may have minimal impact on the crop. In most years it is not common through a lot of the chickpea growing area. Fungicides are available to control Sclerotinia. However, they must be applied prior to the onset of symptoms.