Dry Beans

Generally, rust is not commonly found in Saskatchewan. Bean types differ in susceptibility to rust, and it is much more common in pinto beans than in other types. Symptoms present as pustules on leaves with a yellow halo. Initially, the pustules are full of red summer spores and black winter spores do not develop until late in the season. Plants will become infected if they remain wet for 10 to 18 hours and in moderate temperatures of 16 to 25°C. The disease cycle will repeat every 10 to 14 days, and is usually most severe in late-planted, heavily fertilized beans, with thick canopies.

Early infections that are severe will be devastating to the crop. There are foliar fungicides registered for the control of rust. Infected straw deep plowed into the soil soon after harvest and following a minimum of three-year crop rotation will help decrease the pathogen load for following crops.

Blights are most prevalent in wet, windy weather and can be spread by rain splash. Damaged or cut tissue is particularly susceptible to infection. Blights will result in leaf lesions, defoliation, pod lesions, and shrunken discoloured seed. In Saskatchewan, the two most common blights that will affect dry beans include common blight (Xanthomonas campestris pu. Phaseoli) and halo blight (Pseudomonas syringae pv. Phaseolicola). Common blight is characterized by large, irregularly shaped lesions surrounded by a chlorotic halo. Veins near the lesions are darkened and pods will develop a greasy-looking spot also surrounded by a chlorotic halo that will ooze yellow when wet. Less prevalent is halo blight which starts as small, water-soaked spots that die leaving chocolate brown lesions with a light green halo around them, and pod lesions will ooze a white/cream colour when wet. Pinto, great northern, and red Mexican bean varieties have some tolerance to halo blight.

Both blights are seed-borne, and soil residue-borne, so using disease-free seed is very important. Proper rotation will help lessen pathogen load as well. Streptomycin seed treatment has been used to reduce seed-borne bacterial diseases when importing seed from the United States. However, streptomycin is not registered for use in dry beans in Canada and starting in 2018 there may be further restrictions on importing streptomycin-treated dry bean seed. Copper fungicides can help control blights in season but are ineffective when wet weather favours the development of the disease.

White mould is a serious disease of dry beans in Saskatchewan, as with many crops (such as canola, mustard, sunflowers, and other pulses), meaning the pathogen is often present. Prolonged wet conditions favour the development of the disease, and irrigated beans that are solid-seeded are at high risk. Beans are grown in production systems with more airflow through the canopy to help keep them dry will help reduce the pressure of white mould.

White mould spores land on petal tissue that is dying and falls into the leaf axils on the plant. It then develops a watery lesion and often fuzzy white mould growth. Depending on the timing of the infection, it spreads into the stem and can completely girdle the stem, resulting in poor seed production and premature plant death. Small black sclerotia bodies will form inside the stems that will fall to the soil to produce spores in the following years.

Foliar herbicides are registered for control but should be applied to cover petals before disease symptoms are visibly seen. Production practices that help dry the canopy out will also reduce the virility of the disease.

The pathogen causing Anthracnose in dry beans is specific to dry beans and does not affect other pulse crops. Generally, it is not a common concern in Saskatchewan and can be avoided by using disease-free seed and following a minimum three-year crop rotation, as it is seed- and soil-borne.

Other concerns in dry beans may appear to be diseases but are abiotic or viral and fungicides are ineffective in managing them. It is important to accurately identify the cause of symptoms to ensure the correct management steps are implemented.

Bean common mosaic virus can be spread by infected seed or by sap-feeding insects such as aphids. Affected bean plants are stunted, spindly, have few pods, and may be off-colour. Leaves may be puckered, wrinkled, or elongated. While infections seldom cause plant death, the impact of yield will depend upon the time of infection, with earlier infections having a greater impact.

There is varietal resistance in some bean types, and using clean seed will help prevent issues.

Dry beans are susceptible to damage from sunscald and ozone damage, which results in bronzing. Sunscald is caused by the intense concentration of the sun’s heat on plant tissue. It can affect leaves, stems, or pods and most often is found on lush new growth. Affected leaves look brown and scorched or show white discolouration. The leaf tissue will die and crumble, leaving ragged edges on the leaves. When there are humid, warm, cloudy days followed by bright sun, leaves are most susceptible to damage.

Bronzing occurs when leaves are exposed to ozone. Ozone exposure is related to pollution and lightning events. Similar conditions that favour damage from sunscald also favour damage from higher concentrations of ozone. Leaves will exhibit small reddish-brown flecking on upper surfaces and some pods may show flecking as well. Some varieties are more sensitive to ozone damage, with black varieties more likely to be damaged. Neither condition is considered to affect yield.

Various nutrient deficiencies can appear to be diseases at first glance. Zinc deficiencies may show up in dry beans and can sometimes mimic some diseases or environmental symptoms. Dry beans experiencing zinc deficiencies will have yellowing of the new leaves in between leaf veins that will remain green. The bean plants will have shorter internodes and can appear to be stunted. The top leaves may start to curl under and have a bit of a bronzed appearance.

Alfalfa loopers are occasional pests of dry beans and are more common when grown near alfalfa. Loopers either overwinter in the soil as pupae or adult moths get blown up from the United States in early summer. Generally, there are two or three generations per year, and they may overlap as some may overwinter and some blow in. Loopers may defoliate and clip flowers resulting in yield loss. There is no economic threshold established in dry beans, however, other crops recommend control when populations exceed 15 larvae per metre squared, and heavy defoliation and flower clipping occur.

Cutworms are the larvae of several different types of moths, and they overwinter in the soil as pupae. Larvae emerge from the pupae in the spring and feed on new plants. There are many different species of cutworms in Saskatchewan, and they can affect and eat many different types of crops. Dry beans are no exception and can be cut off below the soil surface by cutworms. Dry beans do not have growing points below ground and therefore cannot regrow after being cut off by cutworms. Crops should be scouted regularly in early spring, and if dead wilted plants are observed, the soil around and up the row from the plant should be examined for the presence of cutworms. The feeding behaviour will vary depending on species, but most feed at night and will be below the soil surface, up to a depth of two inches during the day.

Grasshoppers feed on the foliage of almost any crop, dry beans included. Field edges usually have higher infestations as they move in from field borders. Significant defoliation and yield damage can occur, especially if the other crops surrounding have matured and the dry beans are still immature, which increases the migration of grasshopper populations to the field.

Lygus bugs are not often a significant pest of dry beans, however, if the weather is hot and dry it can favour the buildup of populations and increase the potential for damage to early growth. Lygus bugs feed on plant sap and have piercing, sucking mouth parts. If heavy feeding occurs, they can disrupt the flower set and cause pod abortion. Direct feeding on maturing beans can cause surface pitting, with the potential to affect the quality of the beans. Nymphs tend to be more problematic than adult lygus bugs. A laboratory study in Manitoba found in early reproductive stages, one lygus per inflorescence could reduce yield, primarily by causing pod abortion. At later stages, three lygus per inflorescence was required to affect yield. However, a concurrent field survey of fields with lygus bugs in Manitoba from 2008 to 2010 showed little economic effect of lygus bugs feeding on dry beans.

Potato leafhoppers are small (18 in), pale green, wedge-shaped bugs that move rapidly by jumping. Young nymphs are paler green, lack wings, and walk distinctively sideways when disturbed. Eggs are laid on the plant leaves and hatch in 10 days. Populations increase through the growing season and can get to economic threshold levels by July. Dry bean plants get ‘hopper burn’ from both adults and nymphs sucking sap from plants. Viruses can be transmitted to the plants from their feeding, as well with affected plant tissue becoming dwarfed, crinkled, and curled. Brown areas begin to appear on leaf margins and plants may eventually die.

Treatment is warranted when one leaf hopper per trifoliate leaf is found. A sweep net can provide detection of the hopper but scouting individual plants should be done to determine thresholds. Placing your hand under the leaf will cause them to run to the upper side of the leaf.

Sometimes in warm, moist conditions, fungal pathogens can help control this pest, but predators and parasites tend to play a minor role in helping to control populations.

The adult seedcorn maggot looks like a common house fly and will emerge from eggs in the soil in late May. The adult then lays eggs in moist cracks in the soil that hatch after two to four days when temperatures are 10°C. Larvae are white, tough-skinned, and roughly a quarter inch long. They will burrow into both the seed and the stem of developing dry bean plants, resulting in thinned stands and wilted plants. Damage is worse when germination and emergence are delayed, so ensuring planting as early as possible into a warm, shallow seedbed encourages fast emergence and will help limit the damage. Sprays are not effective and seed treatments are the only control registered to protect against the seedcorn maggot.

Wireworms are small segmented, tan-coloured worms that live in the soil and will feed on a variety of plant hosts. Adult wireworms are beetles that do not do economic damage to crops. Larvae will migrate near the germinating soil in early spring and move back down deeper in the soil as soil temperatures climb, later in the summer. Damage can be seen in the field by wilting plants resulting from wireworms chewing on the stems and may appear similar to cutworm damage. The distinction is that wireworm feeding often looks more like the stem is shredded and it is usually still attached to the roots, whereas with cutworm feeding the stem is cut right off. There are seed treatments available to control wireworms, as they cannot be controlled by foliar sprays as they stay below ground.