Understanding risks of severe phoma stem canker on winter oilseed rape in the UK

Abstract

Phoma stem canker is an economically important disease of oilseed rape in the UK. It is associated with annual yield losses of about £100M and caused by two closely related pathogens: Leptosphaeria biglobosa and L.maculans. Associated with stem-base cankers, L. maculans is considered to be more damaging and is the focus of control. Although generally associated with upper stem lesions, recent UK studies suggest that L. biglobosa can also cause stem-base cankers. Furthermore, L. biglobosa is less sensitive to some triazole fungicides than L. maculans. 

This project aimed to understand stem canker epidemics caused by L. biglobosa and to improve control of phoma stem canker by targeting both causal pathogens. It had four objectives:

1. Determine regional differences in proportions of L. biglobosa in pathogen populations
2. Investigate cultivar resistance against L. biglobosa
3. Investigate effects of fungicides on control of L. biglobosa compared to L. maculans
4. Make recommendations for phoma stem canker control targeted at L. biglobosa and L. maculans (knowledge transfer)

Regional differences

Analysis of air samples, collected from four sites in three cropping seasons, indicated that there were differences between sites and seasons in the timing and patterns of ascospore release. Ascospores were released considerably later at Impington than at the other three sites (Bayfordbury, Rothwell and Eye) in all the three seasons. Ascospores were released later in 2016, when it was hot and dry in the summer, than in 2015 and 2017 at all four sites. There were differences between sites and seasons in the proportions and timings of L. biglobosa and L. maculans ascospores release. In general, there were more L. biglobosa ascospores than L. maculans ascospores in 2016/17 for all sites, whereas there were more L. maculans ascospores than L. biglobosa ascospores in 2015/16 and 2017/18. Both L. maculans and L. biglobosa ascospores were released at similar times at three sites, while L. maculans ascospores were released earlier (mainly in autumn/winter) than L. biglobosa ascospores (mainly in spring) at Rothwell in all three seasons.

Cultivar resistance

Field experiments, using six cultivars with different AHDB Recommended Lists (RL) stem canker disease resistance ratings, showed that there were significant differences between cultivars in the severity of phoma leaf spot and phoma stem canker over the 2015/16, 2016/17 and 2017/18 seasons. In general, resistant cultivars (resistance rating >8) had fewer L. maculans phoma leaf spots and more L. biglobosa phoma leaf spots and less severe phoma stem canker than susceptible cultivars (resistance rating <4). For the two cultivars with the resistance gene Rlm7 (effective against L. maculans), Fencer had fewer L. maculans and L. biglobosa phoma leaf spots, and less severe phoma stem canker than Harper in two out of the three seasons. This suggests that background quantitative resistance affects resistance against both L. maculans and L. biglobosa. With a combination of major gene resistance and quantitative resistance, it is possible to breed cultivars with resistance against both L. maculans and L. biglobosa. 

RL disease observation plot experiments, with 17 cultivars at two sites (Broughton in Hampshire and Morley in Norfolk) in the 2014/15 and 2015/16 seasons, showed significant differences between sites and cultivars in the numbers of L. maculans and L. biglobosa phoma leaf spots, severity of phoma stem canker, and amounts of L. maculans and L. biglobosa DNA in stem base cankers. Susceptible cultivars (resistance rating <4) had greater numbers of L. maculans and L. biglobosa phoma leaf spots, more severe phoma stem canker and greater amounts of L. maculans DNA in stem cankers than resistant cultivars (resistance rating >8). Resistant cultivars had less L. maculans phoma leaf spots, less severe phoma stem canker, smaller amounts of L. maculans DNA but greater amounts of L. biglobosa DNA in stem cankers than cultivars with resistant rating <8. Among the resistant cultivars, one of them showed resistance against both L. maculans and L. biglobosa. Results from controlled environment experiments showed that both L. maculans and L. biglobosa caused severe phoma leaf spots on susceptible cultivars, whereas Rlm7 cultivars (resistant cultivars) showed resistance to L. maculans and variation in susceptibility to L. biglobosa.

Fungicide effects

Field fungicide experiments, in three cropping seasons, showed that Proline (prothioconazole-desthio) and Refinzar (penthiopyrad + picoxystrobin) were equally effective in reducing the numbers of L. maculans or L. biglobosa phoma leaf spots and severity of phoma stem canker on susceptible cultivars, with little or no effect on resistant cultivars. Results of in vitro fungicide sensitivity tests (using individual active ingredients of Proline and Refinzar) with 39 isolates showed that L. biglobosa isolates were less sensitive to prothioconazole-desthio and more sensitive to penthiopyrad than L. maculans isolates. L. maculans isolates collected from Proline-treated plants were less sensitive to prothioconazole-desthio and penthiopyrad than L. maculans isolates collected from untreated plants. There were no differences between L. biglobosa and L. maculans isolates in sensitivity to picoxystrobin. 

Knowledge transfer

Project results were exploited directly in the agricultural industry during the course of the project, through AHDB and industry partners (five oilseed rape breeders, one agrochemical company and one agricultural service provider) of two BBSRC/Innovate UK projects. New findings from this and associated projects were also communicated in six research papers in international journals and as presentations at six international and six national conferences. In addition, results were presented at farm open days and industry events.