Integrated Clubroot Control Strategies for PNW Brassica Producers

1a. Identification of the dominant clubroot pathotypes present in Oregon

Despite the geographic separation of the field sites, only a single pathotype, 16/02/30, was identified using the ECD set (Tables 3 and 4). Three of the four hosts in the ECD set are also part of the Williams differential set, which include ECD 10, 11, and 13 (‘Wilhelmsburger’, ‘Badger Shipper’, and ‘Jersey Queen’, respectively). The ECD pathotype identified in this study would correspond either to pathotype 3 or 6 using the Williams set. Pathotypes 3 and 6 (equivalent ECD designations of 16/15/12 and 16/0/14, respectively) have been identified in Canada, (Strelkov et al, 2006).

An evaluation of thirteen clubroot collections from the west coast of the USA identified the presence of pathotypes 16/02/31 and 16/03/31 (results from Oregon shown in Table 4), which the authors designated as pathotype 7 (Dobson et al. 1983). Williams (1966) also identified pathotype 7 as present in a single sample from California. For a collection to be designated as pathotype 7, a positive reaction for ECD 11 (‘Badger Shipper’) is required, which was not observed in our study, however, one collection had a low-incidence response, and was given the uncertain designation (“?,” Table 4).

Clubroot differential sets have their limitations and results must be interpreted carefully. Within field populations, there is a mixture of virulent pathotypes (Crute et al., 1980) and the differential sets may only identify the dominant pathotype. For example, when single spore isolates were taken from field populations, Xue et al. (2008) identified multiple pathotypes within a population. Different pathotypes can even be found inside a single club (Jones et al., 1982). Some pathotypes occur at a very low frequency and infect only a few plants of a susceptible differential host, resulting in an uncertain reaction. This may be what occurred for collections from sites 1, 2, and 3 that showed low disease incidence for ECD 9 and 11 (Table 3).

Because we identified only a single pathotype in western Oregon, if a cultivar shows resistance on a single farm, it is likely that it would be resistant throughout the region. More detailed results and discussion can be found in the publication “Screening brassica cultivars for resistance to western Oregon clubroot pathotypes” (Heinrich et al., 2017).

1b. Field and greenhouse screening of commercially available cultivars with reputed resistance to the clubroot pathotypes present in Oregon

Clubroot incidence (CI) and DSI for cultivars with purported resistance, and for susceptible cultivars are given in Table 2. At field site 1, high disease pressure was uniformly distributed across the fieldsite, and the CI and DSI for all crop-specific susceptible cultivars were ≥88% and ≥69%, respectively, except in the case of rutabaga. None of the resistant cultivars were completely resistant to clubroot, however, CI and DSI were significantly less than the susceptible check for most crops. For example, at field site 1, 100% of plants of susceptible cabbage, ‘Farao’, had visible clubbing compared to ≤5% of the resistant cabbage cultivars ‘Kilaton’,’ Kilagreg’, and ‘Lodero’.

Field site 2 also had uniform disease pressure and severity across the designated sampling area, however, this trial was not randomized and consisted of unreplicated strips through the field from which subsamples were collected and evaluated. The CI for the susceptible cabbage cultivar Gazelle was 100% compared to ≤15% for the resistant cultivars. No visible clubroot symptoms were observed for the green cabbage cultivar ‘Tekila’.

In the greenhouse study, almost all susceptible cultivars had 100% CI, all with the highest disease rating (R3; Eq.1) while most resistant cultivars showed no visible symptoms. Conditions in the greenhouse represent a worst-case scenario; warm soil temperature, low soil pH, high inoculumn level next to emerging roots, a diversity of isolates from five farms, and ample moisture. Despite conditions being ideal for disease development, most resistant cultivars had a lower frequency of clubbing in the greenhouse than in the field. The difference in CI and DSI between the field trials and the greenhouse suggest that field screening is a better method for evaluating clubroot resistance due to the greater diversity of virulent pathotypes in a field. However, finding a field site with uniform disease pressure, or environmental conditions conducive to clubbing can be a challenge. For example, trials were initiated on five farms in fields with a history of clubroot, and although clubroot was present in all fields, disease incidence and severity were only uniformly distributed across two field sites. For efficiency, preliminary resistance screening should be initiated in the greenhouse to identify resistant cultivars, followed by evaluation of these cultivars in infested fields to better assess resistance potential.

All the resistant cultivars evaluated from B. oleracea showed high resistance to pathotype 16/02/30 except for kohlrabi (Table 2). Cabbage cultivars ‘Tekila’ and ‘Kilaton’, and brussels sprouts cultivar ‘Crispus’ have shown resistance to pathotype 6 (16/00/14) in Canadian field trials (Saude et al., 2012; Sharma et al., 2013). The following cultivars showed high resistance in our study: ‘Clapton’ and ‘Clarify’ (cauliflower; Fig. 3), ‘Lodero’ (red cabbage), ‘Kilagreg’, ‘Kilaton’, and ‘Tekila’ (green cabbages), and ‘Emerald Jewel’ (broccoli). Purple kohlrabi cultivar ‘Azur Star’ showed low resistance (CI of 77%) compared to the susceptible check (CI or 96%). This level of resistance would likely be unacceptable to farmers, and growing these varieties would not contribute to a significant reduction in soil spore concentrations.

Napa cabbages displayed high levels of resistance to clubroot. The cultivars ‘Bilko’, ‘Yuki’, ‘China Gold’, and ‘Emiko’ have previously shown high resistance to pathotype 6 (16/00/14) in Canadian trials (Adhikari et al., 2012; Saude et al., 2012; Sharma et al., 2013). ‘Panda’, was the only napa cabbage with low-resistance in this study, with 60% disease incidence and the highest disease rating (R3) for every infected plant. By comparison the susceptible check had 100% incidence (Table 2).

No resistance was observed for the pak choi cultivars ‘Feng Quing Choi’ and ‘Mei Quing Choi’. ‘Feng Quing Choi’ has not shown resistance to pathotype 6 (16/00/14; Adhikari et al., 2012) and ‘Mei Quing Choi’ has not shown resistance to pathotypes 6 (16/00/14) and 3 (16/15/12; Sharma et al., 2013).

Of all the crops evaluated at field site 1, clubroot incidence was the most variable for rutabaga. For example, the range in CI for ‘American Purple Top’ was 0 to 78%. In the field, average CI was 32% and 26% for ‘American Purple Top’ and ‘Marian’, respectively, but in the greenhouse, no clubbing was observed. This is surprising given that the spore suspension used in greenhouse trials contained clubs from all sites including from field site 1 where clubbing was observed.

This research demonstrated that western Oregon farmers have several commercially available cultivars with resistance to the dominant pathotype in the region. However, to preserve genetic resistance, planting resistant cultivars must be only one part of an integrated clubroot management strategy, rather than the only strategy. Diversity of virulent phenotypes exists within field populations and some of these may be present only at low frequencies. Reliance on resistant cultivars in a short rotation may result in the selection of pathotypes capable of overcoming resistance. For example, frequent planting of resistant canola cultivars has led to the erosion of resistance in Canada (LeBoldus et al., 2012; Strelkov et al., 2016).

More detailed results and discussion can be found in the publication “Screening brassica cultivars for resistance to western Oregon clubroot pathotypes” (Heinrich et al., 2017).

1c. Greenhouse screening of Oregon State University’s archived clubroot resistant germplasm collection for resistance

The broccoli breeding lines CR-1 and CR-2 showed great resistance to western Oregon pathotypes (Table 5). All cabbage breeding lines tested exhibited an infection rate of 33% or more, indicating little resistance. ‘Emerald Jewel’ from Sakata was the only resistant commercially available broccoli cultivar. Many growers did not like its horticultural performance. These breeding lines could be used to create resistant cultivars with more desirable horticultural characteristics from the producer and consumer viewpoint. There is little incentive to breed new European cabbage varieties because multiple seed companies have released cultivars with high resistance to the multiple pathotypes found in Europe, the US, and Canada. This was an initial resistance screening, and these varieties should be evaluated further before forming the basis of a breeding program.

1d. Liming strategies for clubroot control

The goal at Field site 1 was to raise soil pH to approximately 6.8 and more than 7.0 with the 1x and 2x lime rates, respectively, so that we could determine whether pH 6.8 was adequate to control clubroot, as the literature suggests, or whether greater control could be achieved at a higher soil pH. However, too little lime was applied to achieve the target pH (Fig. 4), and it was incorporated more deeply than anticipated. The lime rate was based on a 6-inch incorporation depth, but the lime was incorporated to 8 inches. In addition, the lime was not thoroughly incorporated, and after more than three months, unreacted lime was still visible in the soil profile. As a result, the soil pH at transplant was approximately 6.2, well below the level required to suppress infection and disease development. Even though the pH increased over the growing season, and ultimately reached pH 6.7 and 6.8 for the 1x and 2x lime rates, respectively, lime had no effect on reducing infection rate or disease severity. Clubroot incidence was 100% for the susceptible cultivar ‘Farao’.

At Field site 3, the objective of the study was to evaluate the impact of minimal versus thorough lime incorporation on clubroot incidence and severity. The change in soil pH following the addition and incorporation of lime is given in Fig. 5. Tillage had no effect on the no lime treatments (pH 6.8), but the 2x tillage treatment increased the pH by 0.2 - 0.3 units on the two sampling dates compared to the 1x treatment. At 12 days after incorporation (planting), the pH in the 1x treatment was 7.1 and the 2x treatment was 7.3. At 91 days after incorporation (harvest), the pH in the 1x treatment was 7.4 and in the 2x treatment, 7.7. The higher pH in the 2x tillage treatment is due to the action of the rototiller, which breaks up soil clods and more thoroughly mixes the lime into the soil (Fig. 6). As the result, the lime reacts with a greater proportion of the soil particles, potentially generating fewer soil microsites of pH < 7.0.

Liming reduced disease incidence in both tillage treatments, but reduced severity only in the 2x tillage (Table 6). The no lime 2x tillage treatment reduced both incidence and severity compared to the no lime 1x tillage treatment. Because rototilling did not increase the pH, it may have changed the soil physical conditions, and that change resulted in a decrease in incidence. The difference between the tillage treatments for the limed plots is likely due to a more homogeneous soil pH with fewer microsites of lower pH. Dobson et al. (1983) showed that soil limed to the same bulk pH but with different degrees of lime mixing affected disease incidence and severity. In the less thoroughly mixed soil, they attributed a higher disease incidence to low pH microsites where spore germination was not inhibited.

The change in pH over approximately 400 days in 5 fields following application of 4 ton/acre of Microna Ag lime in August is given in Fig. 7. Site 4 was removed from the study as we were unsure whether sampling was performed in the correct location. In all fields, the highest pH reached was ~7.4, and most fields achieved this pH within approximately 90 days after lime incorporation. For all sites, the average difference between the starting pH and the maximum pH measured was 0.7 units (range 0.4 to 1.1). After 1 year, the average pH was 0.5 units higher (range 0.1 to 1.1) than the starting pH. The data suggests that the pH will be high enough in the spring to control clubroot when fields are limed in the fall. Fall lime applications give farmers greater flexibility in timing; farmers may be too busy in the spring/summer to apply lime shortly before brassica planting or to incorporate the lime thoroughly. In addition, fall applications permit farmers to test soil pH in late winter and apply additional lime before a spring planting if soil pH has not yet reached pH 7.1 or above.

This trial demonstrated that

1. soil pH must reach or exceed the minimum threshold of 6.8 at planting to suppress clubroot inoculation, and to positively impact infection rate or disease severity;
2. lime must be thoroughly incorporated as it only reacts with the soil with which it is in direct contact;
3. estimating the correct rate of lime is critical to achieve the target pH; and
4. lime must be applied long enough in advance to react and reach the target pH before planting. Our results suggest applying finely ground lime products at least one month before planting.