Understanding Sclerotinia infection in oilseed rape to improve risk assessment and disease escape

Abstract

Sclerotinia stem rot in oilseed rape, caused by Sclerotinia sclerotiorum, occurs in the UK at an average incidence of 2%, but there are occasional major outbreaks with high yield loss, which are very difficult to predict. The aims of this Defra LINK - HGCA project were:  [1] to improve the management of Sclerotinia and reduce fungicide use, [2] to assess the potential of polymerase chain reaction (PCR)-based inoculum detection for risk assessment, and [3] to assess the potential of apetally as a disease avoidance mechanism, using near-isogenic lines of oilseed rape.  The apetalous (AP) and full petal (FP) lines used in this project were developed by CPB-Twyford.

The numbers of petals and stamens sticking to oilseed rape leaves are a guide to risk of stem rot developing. In this project, petals, stamens and sepals stuck to leaves and in axils were counted daily on randomly selected plants in each plot. However, the numbers counted did not relate directly to the final levels of stem rot. This was most likely because of large differences in stem rot on lines that had similar petal and stamen stick. The difference in stem rot could be a resistance mechanism, or because sclerotinia is avoided in some way during its life cycle.

The results of testing flower parts for sclerotinia infection showed that AP lines could not avoid infection completely through lack of petals.  The percentage of petals, stamens and sepals infected with Sclerotinia was determined at early, mid and late flower by agar plating tests (ADAS) and also by polymerase chain reaction (PCR) tests (Rothamsted Research). According to agar plate tests, in general, where there was a moderate to high level of FP petals testing positive for S. sclerotiorum, there was also stamen infection on both AP and FP lines, but at lower levels than for petals on the FP lines.  However, more stamens tested positive on the AP lines than the FP lines, e.g. in 2004, at early flower there were 37.5% of stamens testing positive on the AP lines and 24.2 % on the FP lines.  The likelihood is that petals on the FP plants shield FP stamens from airborne spores which are circulating in the crop from the source on the ground (fungal fruiting bodies called apothecia).  The same infection pattern was true for sepals (not tested in 2006), but in general lower levels of sepals tested positive than for stamens.

PCR tests found much higher levels of infection in all flower parts than agar tests but did not detect differences between levels of petal and stamen infection. A quantitative PCR (qPCR) test developed recently shows good potential for assessing stem rot risk from airborne spore inoculum.  The qPCR test gives an indication of the peaks of spore numbers in the air, which correspond to periods of highly infective flower parts. The qPCR test could in future be based upon a simple protocol requiring one or two small spore traps to be operated during flowering and a small number of DNA assays.

Without petals, AP plants are still at risk from infection via stamens. Flower parts infected with
S. sclerotiorum need to stick to leaves for at least a couple of days to cause leaf infection, and in each year of the project, there were similar numbers of stamens stuck on leaves of the AP or FP lines as petals on the FP lines. However, petals were found to be approximately six times as infective as stamens, with about 10% of petals on leaves leading to a lesion, whereas less than 2% of stamens initiated a leaf lesion.

Our results indicate that there is potential for sclerotinia-resistant cultivars, and that apetally does lead to some avoidance of stem rots.  In 2006 the near-isogenic lines AP5 and FP5 differed in stem rot incidence, with 2.2 and 12.6%, respectively.  These lines have a background derived from the cultivar Winner which also had relatively high level of stem rot, 8.8%. However, the near-isogenic lines AP1 and FP1 were derived from Nickel which had low stem rot (2.6%), and both AP1 and FP1 also had low stem rot. Apetally in winter OSR is a multiple recessive gene trait and is difficult to breed. It is unlikely to be commercially viable unless further work is done to develop stable material in a good genetic background.
 
In summary, the key messages are:
1. Petals sticking to leaves indicate a risk of infection, but the numbers of petals sticking may not relate directly to the final stem rot incidence.
2. Petals and stamens can both cause infection, but petals are approximately six times more infective than stamens.
3. A quantitative PCR test shows promise for detecting airborne sclerotinia inoculum within a crop, and could be used in an inoculum based disease forecasting scheme.
4. There is evidence that some winter oilseed rape cvs may be more resistant than others to stem rot, and this needs further investigation.  Different cvs may have different resistance mechanisms.
5. Apetalous lines derived from OSR varieties susceptible to sclerotinia have lower stem rot incidence than full petal lines.  However, it is currently difficult to breed a stable line.