ABSTRACT

The fungal pathogen, Sclerotinia sclerotiorum, has a wide host range and can cause economic damage to oil producing crops such as oilseed rape and sunflower. The pathogen overwinters in the soil as sclerotia, which can germinate to directly infect crop roots or to produce apothecia which release wind dispersed ascospores that can infect leaves and stems. In the UK most disease in oilseed rape and sunflower is caused by ascospore infections. This paper reports the results of a study of the relationship between airborne concentrations of S. sclerotiorum ascospores and subsequent disease development. Sixteen, ten metre square, plots of oilseed rape or sunflowers were inoculated, near their centres, with pots containing sclerotia of S. sclerotiorum. The sclerotia were induced to produce apothecia before placing them in the field. For the experiment with oilseed rape the apothecia were left in the crop for the whole flowering period. Ascospore concentrations in the centre of each plot were continuously monitored using rotating-arm spore traps. In both crops the number of infected plants per plot was related to airborne ascospore concentrations measured several weeks before the appearance of symptoms. Common poppies, Papaver rhoeas, in some of the oilseed rape plots were also infected with S. sclerotiorum and produced sclerotia, suggesting that this weed species has the potential to perpetuate sclerotia in non-host crops in the rotation. The results suggest that airborne monitoring of ascospores could form part of a strategy for stem rot control in oilseed rape. However, routine spore monitoring would require the development of simple- to-use spore detection methods

INTRODUCTION

Stem rot, caused by the pathogen Sclerotinia sclerotiorum, is one of the most damaging diseases of rapeseed in many areas of the World. For example in China it causes average annual losses of about 20%. In the UK the disease is not damaging every year, but when it occurs it can cause significant damage to oilseed rape crops. The disease can be controlled by appropriate applications of fungicides, but in low risk years such applications are not only economically wasteful, but may be environmentally damaging. Thus efficient forecasting systems would be of great benefit to growers, especially in areas where the disease is seasonally intermittent.

S. sclerotiorum has a large host range (Boland & Hall, 1994). It attacks field, forage, vegetable and ornamental crops, trees and shrubs and numerous herbaceous weeds (Zimmerman & Hoes, 1978). In temperate climates, the fungus over-winters as soil-borne sclerotia, survival structures which form in infected tissue. The sclerotia can germinate in the soil producing mycelia which can infect roots directly causing basal stalk rot and wilting. Alternatively, they can germinate carpogenically to produce saucer shaped fruiting bodies, apothecia, up to one centimetre in diameter which release microscopic ascospores that can infect stems and heads (stem and head rot). The ascospores are readily dispersed by wind (Steadman, 1983) and have been detected in the air up to 150 m from the nearest source (Williams & Stelfow, 1979). In sunflower ascospores can infect both stems and heads (Zimmerman & Hoes, 1978). In rapeseed ascospores are not thought to infect plants directly but via infected petals which act as nutrient sources (Davis, 1986). When ascospore infected petals are deposited on leaves, leaf axils or stems the fungus can invade the plant causing disease lesions. The pathogen usually infects main stems but it can also affect leaves, side racemes and pods (Davis, 1986). In both crops sclerotia form in diseased tissue (roots or stems in rapeseed and roots, stems or heads in sunflowers) and can be returned to the soil during harvest, or when an infected plant collapses, to complete the life cycle. Sclerotia have been found to survive in soil for up to 7 years (Gulya et al, 1997), although survival depends on the soil type and depth of burial.

The latent period, that is the time between infection and symptom appearance, for the pathogen in both crops is in the order of a few weeks. Consequently damage has been done before the symptoms are apparent and control measures are then not effective. Thus, to control the disease sprays must be applied about the time infection occurs. Monitoring the occurrence of airborne ascospores offers a method of early detection of infection (McCartney & Lacey, 1991, 1992a,b). In this paper we report the results of experiments to relate airborne ascospore concentrations to subsequent disease incidence in both sunflower and oilseed rape crops.

METHODS

The relationships between ascospore concentration and disease incidence in the two crops were studied in field experiments of similar design, but different treatments. The crops were sown in sixteen 10x10m plots with 5m between the plots and 10m surrounds. The plots were grouped in 4 blocks of 4 randomised plots each with a different treatment (i.e. 4 replicates of each treatment). The treatments consisted of different inoculations with plastic pots containing mature apothecia of S. sclerotiorum. The pots were placed round a 3m diameter circle at the centre of the plot. The pots were watered if needed to prevent the compost drying. In the oilseed rape experiments (1995, 1996 and 1998) the plots were inoculated during flowering and the treatments were: no inoculation, 3 pots of apothecia per plot, 6 pots per plot and 9 pots per plot. The pots were left in the plots for 27 days. In the sunflower experiment (1993) there was a control with no inoculation, in the other treatments the plots were inoculated with 6 pots of apothecia but at the following growth stages 2.4 (plants about 15 cm high) 3.3 (before flowering) and 4.1 (during flowering). The pots were left in the crop for 9 days in each treatment. Apothecia were produced by planting sclerotia, which had been stored at 4°C for at least 4 weeks, 0.5cm deep in John Innes No. 1 compost in round plastic pots. The pots were sealed and stored at 10°C for at least 6 weeks. The pots were then placed under near-ultraviolet light; the sclerotia produced apothecia in about 2 weeks.

Ascospore concentrations were monitored at the centre of each plot during the period that the pots containing apothecia were in the plots. Concentrations were measured using rotating arm spore traps (McCartney et al, 1997) mounted near the top of the crop. As ascospores of S. sclerotiorum are normally released in the morning (McCartney & Lacey, 1991) the spore traps were operated from 0600 till 1800GMT.

In the sunflower experiment disease incidence on stems and heads (proportion of plants showing symptoms) was recorded weekly from the time of appearance of the first symptoms. In the oilseed rape experiments disease incidence was recorded in each plot by harvesting all the plants in a 2m² area at the plot centre.

Figure 1: Disease incidence in the sunflower plots plotted against average ascospore concentration measured during whent the inoculum was in the plots. The symbols represent inoculation at different growth stages: diamonds GS 2.4; squares GS3.3 and triangles GS 4.1.

Figure 2: Disease incidence in the oilseed rape plots plotted against the average ascospore concentration measured during flowering, when the inoculum was in the plots.

RESULTS

The results of the 1993 sunflower experiment and the 1995 oilseed rape experiment are summarised in Figures 1 and 2 which show disease incidence plotted against average ascospore concentrations measured in the centre of each plot.

For the sunflower plots the final disease incidence in the plots tended to increase with ascospore concentration, especially with the first and second inoculations. The results for the first two inoculations appeared to follow the same response line. There was a suggestion that disease incidence resulting from the third inoculation also increased with spore concentration, but that the response line was much shallower than from the earlier inoculations. Disease levels in the uninoculated plots was small (average 6 plants infected) suggesting that most of the inoculum was from the apothecia in the plastic dishes.

In the oilseed rape experiment in 1995 disease incidence at harvest increased with average spore concentrations measured during flowering until reaching a plateau at about 60%, corresponding to an average ascospore concentration of about 150 ascospores m^-3. In the plots with the highest inoculum loads (average concentration of up to 400 ascospores m^-3) disease incidence did not exceed about 60%. This experiment was repeated in 1996 and in 1998 but on both years few ascospores were released and virtually no disease was observed in the plots even though weather conditions were suitable for disease development.

In the 1995 oilseed rape experiment most of the plots were infested with field poppies (Papaver rhoeas). During harvest it was noted that some of the poppies showed typical stem rot symptoms, including white mycellial growths on the stems complete with small sclerotia. Samples taken back to the laboratory confirmed that symptoms were due to S. sclerotiorum.

DISCUSSION

The results from both the oilseed rape and the sunflower experiments suggest that monitoring airborne ascospore concentrations may be useful for predicting the potential for stem rot (oilseed rape) and stem and head rot (sunflowers) development in those crops, provided the weather during the monitoring periods is suitable for infection. However, in sunflowers the relationship between ascospore concentrations and disease potential may be dependent on the timing of ascospore release. This may be due to the susceptibility of different parts of the plant because in this experiment, as in earlier ones (McCartney and Lacey, 1992a, b), stem lesions occurred when ascospores were in the air before flowering while head lesions were more common when the ascospore peak was during flowering. These experiments also suggested that in years when disease was severe there appeared to be a correlation between disease incidence and the concentration of ascospores in the air about five weeks previously (McCartney & Lacey, 1992b). In oilseed rape S. sclerotiorum infections are mainly initiated via ascospore infected petals deposited on leaves. In Canada, a scheme has been developed to monitor infection of oilseed rape crops by detecting ascospores on petals (Morrall & Thomson, 1991). However, it is not unreasonable to assume that ascospore loads on petals will be related to airborne concentrations, thus airborne measurement of inoculum could be an alternative to petal monitoring if simple easy-to-use methods became available.

In the oilseed rape experiments poppies growing in the plots also became infected with S. sclerotiorum and produced small viable sclerotia. In the UK in recent years there has been a tendency to reduce weed control in cereals, which are nearly always grown in rotation with oilseed rape. If field poppies, or other weed species, are susceptible to the fungus then they have the potential to act as a source of sclerotia in non-susceptible crops grown in rotation with susceptible crops.

CONCLUSIONS

Although these experiments suggest that monitoring of airborne inoculum of S. sclerotiorum is potentially useful in forecasting the likelihood of the development of damaging epidemics of stem rot much work will be needed before practical systems can be produced. Current spore sampling methods are based on microscopic or cultural identification of spore catches. These take both time and experienced personnel. Novel methods for quantifying airborne spores based on immunological or molecular techniques are being investigated (McCartney et al, 1997) which should result in “user friendly” rapid ascospore detection methods. More detailed information is also needed on the relationships between ascospore concentrations and disease development to define inoculum action threshold values. Nevertheless, inoculum quantification is potentially useful in the management of diseases caused by S. sclerotiorum in a number of susceptible crops.

ACKNOWLEDGEMENTS

IACR-Rothamsted is supported by a grant in air from the UK Biotechnology and Biological Sciences Research Council.

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