ABSTRACT

Sporulation rates of A. brassicae and A. brassicicola at high and low temperatures are an important factor limiting the occurrence and spread of each species at different stages in the cropping season. Controlled environment studies have identified some of the main climatic variables involved in sporulation by A. brassicae and A. brassicicola. At optimal temperatures, sporulation of both pathogens requires at least 12h with a minimum relative humidity of 87%. However, at sub-optimal temperatures A. brassicicola required significantly longer durations above this humidity for spore production to occur. Models describing the effect of temperature and vapour pressure deficit (vpd) have been developed using these experiments for both pathogens. These relationships have been validated using observations collected under field conditions. Observed spore production has been compared over a range of model outputs derived under varying conditions of temperature and vpd duration for both pathogens. This analysis compared observed and expected spore production over three years of spore trapping data collected in the field from B. napus crops infected with both pathogens. The paper describes the model and tests conducted on the validity of this relationship using spore trapping data.

INTRODUCTION

Dark leaf spot caused by Alternaria brassicae (Berk.) Sacc. and Alternaria brassicicola (Schwein.) Wiltshire is a serious disease of brassica crops world-wide (Humpherson-Jones and Phelps, 1989). Economic damage resulting from this disease is particularly serious on seed crops. In Brassica oleracea, yield losses of 70% resulting from infection by A. brassicicola have been reported (Neergaard, 1945). Yield losses greater than 50% have been reported in oilseed rape crops infected with A. brassicae (Daebler et al., 1986).

Relationships between weather and A. brassicae sporulation and release in the air within an infected crop of oilseed rape indicated that warm conditions following periods of wetness were favourable (Louvet & Billotte, 1964). Measurement of sporulation under field conditions has been difficult as this will be affected by the degree and intensity of factors influencing spore release. Environmental factors governing the degree of spore release have not been measured. Spore release during periods when there is no rainfall is inhibited by high humidity and triggered by falling humidity (Humpherson-Jones, 1992). Measurement of sporulation in the field using spore trapping therefore depends on the degree of spore release.

Both species have the potential to complete their full life cycle rapidly. Prediction of disease spread is, therefore, of critical importance in the timing of control measures. The present paper describes the use of controlled environment data described in previous studies (Humpherson-Jones & Phelps, 1989) in the construction of weather related models for predicting sporulation of both dark leaf spot pathogens. Evaluation of these relationships using spore trapping data collected under field conditions is also reported.

EXPERIMENTAL

Spore trapping studies in the field

Plots of oilseed rape (cv.Jet Neuf) measuring 15 m x 15 m. were direct drilled in beds with a 17 cm row spacing in the autumn to produce seeding crops. Crops were produced during 1982, 1983 and 1985 at HRI Wellesbourne. Volumetric spore traps (Burkard Scientific Ltd., Rickmansworth, Hertfordshire, England) were operated continuously (Hirst, 1952) within the plot to sample the air at 10 l min^-1 from mid May to late July from 1980 to 1985. Traps were placed in a 2m diameter clearing in the centre of the crops with the orifice 40cm above the ground. Air temperature and relative humidity were recorded within the crops at a height of 30 cm with a screened bi-metal strip/hair thermohygrograph. Leaf wetness was recorded at a similar height using a Meteorological Office surface wetness recorder. Rainfall was recorded on open ground 2-3 m outside the crops with a tilting syphon rain guage. Hourly records of spore concentrations and weather variables were stored on computer.

Model Development

Data from controlled environment studies relating temperature and sporulation times (Humpherson-Jones and Phelps, 1989) were used as a basis for the development of models predicting spore production by A. brassicae and A. brassicicola. A temperature-dependent sporulation rate function was obtained from these data and applied to fluctuating temperatures encountered in the field. Integration of this function over time indicated when the process of spore production was complete (Rabbinge, et al. 1989). It was assumed that under appropriate conditions sporulation would proceed at any time of day and be independent of previous sporulation events. The following assumptions were also appropriate:

• A vpd of less than or equal to 3.0 continuously (equivalent to a RH of 87% at 20⁰C and 76% at 10⁰C) was required for sporulation to proceed as this had been observed in the controlled environment experiments.

• Any breaks in these conditions would cause sporulation to recommence from zero.

Using field data to test assumptions

Using spore trap records, in conjunction with environmental data, within crops of B. napus infected with A. brassicae or A. brassicicola, the effect of vpd threshold and duration of dry break (within spore production periods) was assessed. Most spore production was observed during night periods (approximately 21.00h to 09.00h the following day). It was assumed that spores produced during the night were released (if produced) the following day. The number of false positive (sporulation predicted but not observed) and false negative predictions (sporulation not predicted but observed) of sporulation were calculated for durations of vpd less than either 3 or 4 in combination with dry breaks of either 0, 2, 4 or 6 hours. Calculations of false positives and negatives used spore and environmental data collected during years 1982, 1984 and 1985 in oilseed rape crops. Prediction of sporulation was based on a model score of 100 which denoted that environmental conditions were fulfilled for spore production to have occurred. Observed sporulation was recorded as having occurred when the total number of spores trapped per day exceeded 10.

RESULTS

Diurnal periodicity of spore release by A. brassicae and A. brassicicola

The diurnal periodicity of A. brassicicola spore release during a 24 hour period in July 1985 from B. napus crops and the corresponding temperature and humidity conditions is shown in Figure 1. A similar pattern in numbers of spores of A. brassicae trapped over an equivalent period was observed (data not presented). There was significantly high spore numbers of A. brassicicola spores trapped from 0.700 h until 21.00 h during this period. Temperature increased and humidity decreased over this period. Significant numbers of A. brassicicola spores were trapped before 08.00h. Maximum spore counts were observed during 13.00 h – 15.00 h. Similar patterns of spore release by both pathogens was observed over the entire season although the spore numbers were lower during the early part of the season when disease development was restricted within the plots.

Prediction of sporulation by A. brassicae and A. brassicicola

Time to 50 % spore production was plotted against each test temperature (Figure 2). The sporulation rate (data not presented) by A. brassicae was optimal between 18°C and 24°C. Sporulation was inconsistent at 26°C and spores formed at this temperature were considered unviable. Therefore it was assumed that the rate would be 0 above 25°C. The rate curve for A. brassicicola appeared to reach an asymptote at 20°C (data not presented) following a sharp increase. Sporulation by A. brassicicola was observed over a greater temperature range. However, time to 50 % spore production was greater over the lower temperature ranges (Figure 2). No sporulation by either pathogen was observed in additional

controlled environment experiments conducted below 5°C over 48 h time periods.

Using field data to test assumptions

The numbers of false positive and false negative predictions of spore production for both A. brassicae and A. brassicicola were compared with spore trapping observations taken during July in years 1982, 1984 and 1985 in a seeding oilseed rape crop infected with both pathogens (Table 1). The number of false negative predictions for both pathogens decreased when increasing duration of dry break was assumed not to stop spore production. This was especially evident for observed and predicted sporulation by A. brassicae. There were fewer false negative predictions of sporulation when the threshold vpd above which sporulation was assumed not to occur was increased to 4. This effect was more pronounced for A. brassicae.

Table 1 Comparison of observed and predicted sporulation

CONCLUSION

The results indicate that models developed using controlled environmental data can be successfully used to predict sporulation under field conditions. Using spore trapping data it was shown that sporulation was unaffected by breaks in conducive conditions. This preliminary analysis indicated that higher vpd thresholds may be required to inhibit spore production although this point would require further experimental work. Environmental conditions during the day period would therefore appear to be an important limiting factor for spore production and an important factor in the epidemiology of the disease.

ACKNOWLEDGEMENTS

This work was funded by the Ministry of Agriculture, Fisheries and Food.

REFERENCES

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