Dr. Syama Chatterton, Plant Pathologist, Lethbridge Research Centre, Agriculture and Agri-Food Canada
Aphanomyces root rot is historically the most destructive disease of peas. Although reported in Manitoba for the first time in 1997 and into the early 2000s, it has recently made a resurgence and was reported in Saskatchewan for the first time in 2012 and Alberta in 2013. Since that time, prairie-wide surveys have revealed that it is widespread across all pea- and lentil-growing regions of the prairies. Wet and warm soil conditions favour the disease, and thus the wet growing seasons of the early-mid 2010s likely caused the widespread explosion of the disease.
Aphanomyces root rot is caused by Aphanomyces euteiches. This micro-organism belongs to a group called oomycetes or water-moulds because, unlike fungi, there is a portion of its life cycle that is dependent on water. Zoospores, microscopic swimming cells (Figure 1), are released from resting spores (oospores, Figure 2) under high soil-moisture conditions when a host crop is present. The zoospores can swim through free water in soil pores to find and attach to a host root. The zoospores then germinate to produce mycelia (thread-like strands) that infect host roots and quickly spread throughout the root system. When root nutrients have been used up, or environmental conditions become inhospitable, the pathogen initiates sexual reproduction to produce thick-walled oospores. Oospores survive in soil and plant debris in a dormant state for many years and can withstand freezing and dry conditions. Thus, A. euteiches overwinters exclusively as oospores. Oospore germination and infection can occur at any time over the growing season, as long as soil moisture is adequate. Although zoospores are the propagules of A. euteiches that directly infect the host, their quantity is dependent on the number of viable oospores within the soil. Therefore, oospores are considered the primary drivers of disease and quantification of oospores in the soil will facilitate disease prediction.
Symptoms and host range
Roots infected by A. euteiches appear honey-brown in colour, with symptoms appearing on lateral roots very quickly after initial infection (Figure 3). However, it is difficult to catch the disease at this stage, as infection spreads through the roots quickly. The root cortical (or outer) tissue becomes soft and dark brown in colour as the disease progresses, and infection moves into the main taproot and epicotyl (portion between the seed and green stem), which becomes pinched, honey-coloured and soft (Figure 4). This is the stage where Aphanomyces root rot is the most recognizable and can be distinguished from other root rot pathogens. However, this stage is also short-lived, and in later stages, when infected plants are pulled from the soil, the cortex becomes easily sloughed off and remains behind in the soil. Oospores are produced in these outer infected tissues. In later stages, the epicotyl and taproot also become dark or blackened, usually due to infection by other organisms, such as Fusarium and Pythium species (Figure 5). When an infection is severe, shoots become wilted, chlorotic and stunted, with leaves of infected plants progressively yellowing from the bottom of the shoot upward (Figure 6). Shoots are particularly affected if infection occurs early in the season. Because the root system becomes decayed, nodule production is also affected, which further exacerbates symptoms, as plants will not be able to fix nitrogen needed for growth. As a result of both root infection and nodule loss, infected plants produce fewer pods, a reduced number of seeds per pod, or may even die before pod development. The degree of yield loss is largely dependent on the environment. Very wet conditions in spring (approximately 4–6 weeks after seeding), followed by dry conditions, often result in the most significant yield losses, as plants will have a diminished root system that cannot deal with drought stress.
In low to moderately infested fields, the disease is first evident as yellow patches in low-lying areas of the field, or in areas where water accumulates, like water tracks or seepage areas (Figure 7). It is important to note these test areas, as the pathogen can quickly spread across the whole field during periods of flooding or water-saturation of the field. An inoculum density of 100 oospores/g soil is the threshold needed to cause moderate to severe root rot. Oospore levels can increase quickly in these infested patches. A field with a low inoculum level, or a few small patches, can produce sufficient inoculum to destroy a subsequent pea crop, particularly if wet conditions persist in favouring infection. In highly infested fields, the pathogen tends to be distributed fairly uniformly across the field, and the entire field can show yellowing. However, areas prone to water collection will be worse.
Aphanomyces has a host range confined to the legume family. In inoculation tests, A. euteiches could infect peas, alfalfa, snap beans, dry beans, red clover, lentils, vetches and many different weed species. Faba beans, chickpeas and soybeans may show little to no infection but is dependent on variety. There is evidence that some non-legume plants, such as spinach, oats and red-root pigweed, can be infected by A. euteiches and this can potentially be exploited to reduce oospore levels in soil. There is also some degree of host-specialization of different isolates of the pathogen. Isolates from peas can infect other legumes, including alfalfa and lentils, but most isolates from beans and red clover do not infect peas. Furthermore, races of the pathogen have been described for alfalfa, but not for peas. For alfalfa, there are two distinct races of A. euteiches with different varieties of alfalfa displaying resistance to race 1 or 2.
Disease Management and Research
Aphanomyces root rot remains very difficult to manage because the oospores are long-lived and soil-borne. Intego Solo (a.i. ethaboxam) is the only product currently registered for early-season suppression of Aphanomyces root rot but does not provide full-season control. The most effective way to reduce losses caused by this disease is to avoid planting peas or lentils in moderate to highly infested fields. Many labs in the prairies offer soil testing services for the presence/absence of A. euteiches based on DNA screening of root or soil samples, but to date, do not provide information on infestation level. If a field tests positive for Aphanomyces, crop rotation away from peas or lentils for 6–8 years or longer is required to reduce oospore levels below the threshold. Legume species such as soybeans, chickpeas, fenugreek, lupins and some faba bean varieties are resistant and can be good options to replace peas. In contrast, lentils, alfalfa and vetches are susceptible to the disease and would contribute to increasing oospore levels in the soil. There are currently no cultivars of pea with resistance to Aphanomyces root rot.
Aphanomyces root rot is a relatively new disease to the prairies, so research into managing this disease on the prairies also started relatively recently. Research at the University of Saskatchewan is underway to develop cultivars with partial resistance to the disease. There are several research projects aimed at agronomic practices that may mitigate impacts of Aphanomyces root rot, which include evaluation of various seed treatments, soil amendments (e.g. liming), crop rotation and use of Brassica, rye and oat cover crops or intercrops. Significant research effort into developing a quantitative test for measuring oospore levels in the soil and predicting disease risk is also ongoing.