Phoma resistance in oilseed rape feels the heat

Resistance to phoma stem canker in oilseed rape (OSR) is highly complex, according to results published in a new AHDB final report.

The research has improved understanding of how air temperatures influence the effectiveness of disease resistance during the growing season. Details on which genes and mechanisms are most affected by elevated temperatures have also been revealed.

The structure of pathogen populations across the country was also found to vary significantly. Information on this variability could be used to help target breeding efforts and, ultimately, help farmers make better variety choices.

About phoma stem canker

Phoma stem canker is caused by two closely related fungal plant pathogens, Leptosphaeria maculans and L. biglobosa. As the former is thought to be associated with more severe epidemics, it was studied in the research.

Disease resistance ratings for stem canker are published on the AHDB Recommended Lists (RL), using a 1–9 scale (where 9 is resistant). There is a wide range of resistance on the 2019/20 RL – with ratings from 3 to 8.

There are two types of resistance against the phoma stem canker pathogen: single resistance genes (R gene-mediated resistance), which operate in leaf tissues in autumn; and, quantitative resistance (QR), which operates in the leaf stalk and stem tissues.

It was already known that increased temperature renders some R genes ineffective (see Figure 1) and decreases the effectiveness of QR. As climate change is predicted, on average, to make the climate warmer, it was deemed essential to better understand phoma resistance mechanisms in OSR, as well as the variability in the UK phoma pathogen population.

The phoma population

Susceptible OSR variety ‘Drakkar’ succumbs to phoma because it has no R gene. Its vulnerability means it can be used to capture snapshots of local L. maculans populations. During the research, Drakkar was grown at several sites, over three growing seasons, with pathogen isolates collected each autumn.

A set of OSR varieties, with known R genes, was then used to classify each population (from each site and season), based on how the plants reacted to infection. Significant differences in race structure, for both seasons and sites, were found. For example, there were regional differences in the ability of the population to overcome the R gene Rlm4. The results also suggested that Rlm2 and Rlm9 are not effective against L. maculans. On the other hand, Rlm7 provided consistent resistance, except in the 2011/2012 season. Even in this season, only 3% of isolates were able to cause disease. Although this gene was found to be highly effective, the population requires monitoring to detect any significant changes to resistance.

Field experiments also confirmed that R genes combined with QR can improve the effectiveness of resistance in different environments.

Temperature and resistance

To develop a better understanding of the influence of temperature on resistance, the researchers used a controlled environment (CE) to test how the various resistance mechanisms performed at 20°C and 25°C.

In an experiment where cotyledons were infected, it was found that some R genes were sensitive to temperature, whereas others were not. At elevated temperatures, QR was also found to play a role in how effective R genes were.

Two cultivars with Rlm1 and two with Rlm 4 were used in leaf-stalk inoculation studies, along with a susceptible control. At 20°C, these cultivars had no/limited stem canker symptoms. However, at 25°C, differences were observed – there were no/limited stem canker symptoms on the stems of DK Cabernet (Rlm1) and Adriana (Rlm4), which also had QR. However, stem cankers developed on Bilbao (Rlm4) and Capitol (Rlm1), which did not have QR. Results from the research show clearly that QR affects how R genes perform at increased temperatures (see Figure 2).

Based on novel methodology, developed as part of the research, the researchers also found that R gene-mediated resistance, which was already known to operate at the leaf-infection stage, also operates at the stem-colonisation stage.

The work has radically improved understanding of resistance to phoma in OSR. The results, published in several high-profile journals, will help focus breeding programmes. In the future, it is hoped that varieties with durable (combined R and QR resistance) and temperature-resilient resistance will be produced.

Further information on the disease can be accessed via ahdb.org.uk/phoma

Notes to editors

This article is based on Final Project Report 606: Understanding resistance to decrease risk of severe phoma stem canker on oilseed rape. This is the final report of a 48-month project (RD-2009-3676) that started on 1 January 2011 and was led by the University of Hertfordshire. The work was funded by BBSRC LINK, Cadacre Agricultural Trust, Felix Cobbold Trust, Perry Foundation and a contract for £117,400 from AHDB. The total cost of the project was £1,127,000. The publication of the final report was delayed to facilitate the preparation and publication of several high-profile scientific publications.

Figure 1. It was known that Rlm6 is not as effective against phoma at increased temperatures. This applies to both R genes operating in the leaf lamina (top) and QR operating in the leaf stalks and stem tissues (bottom)

Figure 2. Phoma stem canker symptoms on cultivars with different types of resistance (July 2012)