Crop Protection

Weed control

Dry bean is a short-stature crop that does not often form a complete canopy until mid-July. It is a very poor competitor to weeds and must be grown on fields that are relatively weed-free. Control of perennial weeds such as Canada thistle and sow-thistle must be done the year before dry bean production.

As dry bean is not seeded in Saskatchewan until May 25, there is time to apply a pre-emergent burn-off herbicide to control early-emerging and perennial weeds before the crop emerges. Bean is susceptible to the soil residues of many of the herbicides used for broadleaf weed control prior to seeding, with the exception of glyphosate, Gramoxone, and Pardner/Koril. This may make the control of glyphosate-resistant canola volunteers more difficult.

Plants in the wheel tracks of ground sprayers/applicators in emerged crops may be later maturing than the rest of the crop. This can seriously complicate the timing of harvest. Tramlines should be considered in narrow-row bean production to reduce this problem.

For an up-to-date list of registered herbicides, consult the product labels or the Guide to Crop Protection. Be sure to check the herbicide labels before application, because some products are not registered on all classes of dry bean. For example, Pursuit is only registered on pinto, pink, and red varieties.

Early application of herbicides is important to control weeds and improve yields. Green weeds at harvest time can lead to staining and downgrading of the crop. Successful control of broadleaf weeds is a very important factor in narrow-row bean production, as no between-row cultivation is possible. Some solvent burning may occur on the crop leaf surface, but experienced bean growers have found that often a second application of post-emergent broadleaf weed herbicide is required for improved yields and to reduce harvest problems with weeds.

Dry bean seedlings are very sensitive to spray drift from herbicides for broadleaf weed control found in Herbicide Groups two and four, and from glyphosate, Liberty, and Amitrole.

Pre-harvest perennial weed control with glyphosate is registered in dry bean. Application should be done when the seed moisture is below 30 per cent, the stems are green to brown, and the pods are mature, with 80 to 90 per cent of leaves already dropped. Do not use planting seed that has received a pre-harvest application of glyphosate.

Disease control

Seedling diseases

Dry bean suffers from a complex of seed rot and seedling blight diseases caused by Fusarium, Rhizoctonia and Pythium. These disease pathogens are present in all Saskatchewan agricultural soils and can infect and kill individual seedlings from the germination to the pod-filling stage. Lesions develop at the base of the stem, causing discolouration and constriction of the stem. Diseased plants turn yellow and die. Usually, only scattered plants are infected. However, these diseases can cause economic loss. Fusarium is more serious in dry years and plants suffering from the disease show stunted yellowed leaves, form fewer pods and produce smaller seeds.

The best way to prevent seedling disease is to observe a proper crop rotation where dry bean is planted no more than once in four years. The use of vigorous seed can reduce the risk of seed rots and seedling blights.

A number of seed treatment fungicides are registered on dry bean for the control of seedling diseases. For more information about these products see the Guide to Crop Protection. Some products are available to commercial seed treaters only.

These fungicides can impact the viability of nitrogen-fixing inoculant products. The use of an in-furrow granular inoculant can minimize contact with treated seed. Always check the inoculant label or contact the manufacturer for more information on the compatibility of seed treatment fungicides and inoculants.

Viral diseases

Rust

Rust infection is favoured by moderate temperatures (16 to 25 C) and 10 to 18 hours of leaf wetness. Infection and pustule development is greatly reduced if dry and warm conditions occur.

Bacterial blight

Several species of bacteria can cause blight in dry bean, but the two most common are common blight (Xanthomonas campestris pv. phaseoli) and halo blight (Pseudomonas syringae pv. phaseolicola). These foliar diseases are favoured by wet, windy weather, and can be spread through the field by mechanical operations and hail. Symptoms include lesions on leaves and stems, which start as small, water-soaked spots. These small spots coalesce into large brown lesions and are surrounded by a yellow margin (halo). A crop rotation with at least two years between bean crops will help to reduce the risk of the disease.

Sunscald or herbicide solvent burn can cause similar symptoms as bacterial blights. Make sure the problem has been properly identified before taking control measures.

Common and halo blights are seed-borne and residue-borne. The use of bacteria-free seed is essential to prevent the disease. 

Streptomycin seed treatment is used to reduce seed-borne bacterial diseases. Rules pertaining to the importation of bean seed treated with streptomycin were initially addressed in the 2003 crop year. Special labelling requirements remain in effect and all seed bags must be tagged with instructions on the proper handling of streptomycin-treated seed.

Copper-based foliar fungicides are registered for control of bacterial blight in dry bean.

White mould

White mould (Sclerotinia sclerotiorum) is a major foliar disease of dry bean and can cause severe crop loss. White mould is almost always present on irrigated dry bean. Narrow-row production can also increase the risk of white mould due to limited air movement under the crop canopy. The disease is most common in fields that were recently planted to other susceptible broadleaf crops such as canola, mustard, pulses, or sunflower. Resultant crop losses are heaviest if wet weather occurs during the flowering stage.

Disease spores germinate on fallen flower petals and rapidly produce lesions that begin as small water soaked spots, often first appearing at branches of the stem. The lesions rapidly expand and are often covered by a white, cottony growth.

Whole plants can be killed if the lesions girdle the main stem. Infected pods produce small, discoloured seed. Small, black, hard bodies (sclerotia) form within the lesions and fall to the ground, where they can remain dormant for up to five years. When conditions are right, these black bodies develop spore-producing structures.

To reduce the risk of white mould, avoid crop rotations with a history of susceptible broadleaf crops within the three previous years and avoid planting bean crops adjacent to fields infected with the disease in the previous year.

Begin field scouting for white mould just prior to the flowering stage of the crop. Early signs include the presence of individual yellow or wilted stems and very small mushroom-like, spore-producing structures on the soil surface under a thick canopy.

Check the product label for proper recommendations concerning time of application. Budget for an application of fungicide for the control of white mould.

Anthracnose

Anthracnose (Colletotrichum lindemuthianum), a serious disease of dry bean in other areas of the world, is not a problem in Saskatchewan. It is seed-borne and residue-borne, and can be avoided by using disease-free seed and a crop rotation with at least three years between bean crops. The fungus that causes anthracnose on dry bean is different from that causing anthracnose on lentil.

Insect Control

Grasshopper

Grasshopper damage occurs on leaves and can be severe on field edges after adjacent crops have matured.

Cutworm

Cutworms often feed at night, chewing through the seedling stem just below the soil surface. They do most of their damage in late spring and will attack many broadleaf crops, including dry bean. Red-backed and Pale Western cutworm are two common species affecting dry bean in Saskatchewan.

Bean crops should be scouted regularly. If wilted, dead, or cut-off seedlings are noticed, dig around the roots of the plant to check for the presence of cutworms.

Wireworm

Similar to cutworm, wireworms cause damage to seedlings in the spring. Feeding damage results in wilted plants but, unlike cutworm, the seedling generally remains attached to the root.

Seedcorn maggot

Seedcorn maggots can attack bean seed after planting. They prevent germination or weaken seedlings. The yellowish white maggot can be found burrowing in the seeds or emerging stems.

Seedcorn maggots are usually most severe in wet, cold seasons and when delays in germination and emergence occur. If maggot pressure is extremely high, replanting with treated seed may be the only option. Seedcorn maggot infestations are generally not widespread and can be variable within fields and between adjacent fields.

Potato leafhopper

Adult potato leafhoppers are slender, 2.5 mm (1/10 in.) long, light green, and have short, bristle-like antennae. The females lay eggs on plants, and the nymphs hatch in about 10 days. Both adults and nymphs suck sap from the vascular tissue of the plant. By July, a sufficient population of leafhoppers can be present to cause stunted plants that may eventually die. The economic threshold for insecticide application is one leafhopper per trifoliate leaf.

Lygus bug (Tarnished plant bug)

Hot, dry weather favours an increase in the population of lygus bug and increases the possibility of damage to bean seedlings. Lygus bugs have piercing mouth parts that suck sap from the plant. Several insecticides are registered for the control of lygus bug on bean.

Certain groups of fungicides, such as strobilurins in Group 11, have high risks of developing resistance if used inappropriately. Development of resistance of several fungal pathogens to this group of fungicides has been reported and is of great concern. No more than two applications per year of any strobilurin fungicide should be made to the same field, as disease resistance could develop. The continuous use of strobilurin fungicides without fungicide rotation greatly increases the threat of the disease resistance.

Sunscald

Following sunny, hot weather, dry bean can exhibit small, brown, irregular spots between the veins. The spots expand, often leaving large areas of dead tissue between healthy green veins. Bronzing occurs when upper leaf surfaces are covered with small golden-brown spots, which make the leaf appear bronze in colour. No controls are suggested.

Desiccation

Reglone® desiccant is registered for use on dry bean. The product is used to evenly dry-down the crop and weeds to hasten harvest. It will not speed the maturity of the crop. Apply it to dry bean when 80 to 90 per cent of the leaves have dropped and 80 per cent of the pods have turned yellow. For more information on desiccation of dry bean, consult the product label.