CSIRO PUBLISHING www.publish.csiro.au/journals/ajar Australian Journal of Agricultural Research, 2003, 54, 599–611 Effect of climate variability on event frequency of sorghum ergot in Australia Enli WangA,C, Malcolm RyleyB, and Holger MeinkeA AAPSRU, DPI/CSIRO/NRM/UQ, PO Box 102, Toowoomba, Qld 4350, Australia. BQueensland Department of Primary Industries, PO Box 102, Toowoomba, Qld 4350, Australia. CCorresponding author; present address: CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia; email: [email protected] Abstract. The significant effect of ergot, caused by Claviceps africana, on the Australian sorghum industry, has led to considerable research on the identification of resistant genotypes and on the climatic conditions that are conducive to ergot outbreaks. Here we show that the potential number of monthly ergot events differs strongly from year to year in accordance with ENSO (El Niño–Southern Oscillation) related climate variability. The analysis is based on long-term weather records from 50 locations throughout the sorghum-growing areas of Australia and predicts the potential number of monthly ergot events based on phases of the Southern Oscillation Index (SOI). For a given location, we found a significant difference in the number of potential ergot events based on SOI phases in the preceding month, with a consistently positive SOI phase providing the greatest risk for the occurrence of ergot for most months and locations. This analysis provides a relative risk assessment for ergot outbreaks based on location and prevailing climatic conditions, thereby assisting in responsive decision-making to reduce the negative effect of sorghum ergot. AR02198 EveE.et ntWalang f. requency of sor ghum ergot Introduction pollen viability (Downes and Marshall 1971; Brooking Ergot, caused by Claviceps africana Frederickson, Mantle & 1976, 1979), and then predispose sorghum plants to ergot DeMilliano, is a non-systemic disease of the ovaries of infection (McLaren and Wehner 1992; McLaren and Flett Sorghum species, including Sorghum bicolor (L.) Moench. It 1998). In the main sorghum-growing regions in Australia is now endemic in all the major sorghum-producing this situation occurs only for very early- or late-planted countries of the world, including Australia, where it has had sorghum. Temperature and humidity during flowering are the a significant effect on some sectors of the sorghum industry, more important factors influencing infection (McLaren and particularly seed production and the use of sorghum as feed Wehner 1990; Wang et al. 2000a). To account for the effect grain (Ryley et al. 2001, 2003). The high susceptibility of of temperature during the flowering period on ergot severity, male sterile A-lines in hybrid seed production blocks has Wang et al. (2000a) developed an infection factor based on necessitated the regular use of triazole fungicides (Ryley hourly temperatures during that period. They showed that an et al. 2001). The alkaloids in the fungal structure infection event was likely to occur if the infection factor (sphacelium/sclerotium) that replaces the ovary have been during the flowering period was >0.3 and the mean relative found to be detrimental to livestock (Blaney et al. 2000), and humidity at 09 00 hours was >70%. this has affected the acceptability of ergot-contaminated A climatic analysis on the mean number of potential sorghum grain by many livestock producers. monthly ergot outbreaks indicated that there were increasing Although genotypes and hybrids of S. bicolor have been gradients of ergot events in eastern Australia from south to shown to vary in their ‘resistance’ to C. africana (McLaren north as well as from west to east between December and 2000; Dahlberg et al. 2001), there are no commercial ergot- March, when most commercial grain sorghum crops are resistant hybrids. Fertilised ovaries are generally resistant to flowering. Later in the season (April–May), when some late- infection by conidia of C. africana (Bandyopadhyay et al. planted crops may be flowering, the number of potential 1998), so ergot infection is strongly influenced by the monthly events increased, particularly in the southern areas. amount of viable pollen, and by humidity and temperature The lowest number of events occurred between September during the flowering period (McLaren and Wehner 1990, and December when few commercial grain sorghum crops 1992; McLaren 1997; McLaren and Flett 1998). Low night are flowering. The temporal and geographic distribution of temperatures, with a daily minimum temperature of <13°C the number of potential events and of ergot severity was during the flag leaf stage, can induce male sterility, reduce closely related to that of relative humidity during the © CSIRO 2003 10.1071/AR02198 0004-9409/03/060599 600 Australian Journal of Agricultural Research E. Wang et al. flowering period (Wang et al. 2000b). Further, it was found Table 1. Source of climatic records used in the analysis that the number of ergot events differed strongly from year to year, depending on the season type. Stn no. Station name Latitude Longitude It has been well established that eastern Australia is 033002 Ayr DPI Research Stn –19.62 147.38 strongly affected by the El Niño–Southern Oscillation 039006 Biloela DPI –24.38 150.52 phenomenon (ENSO), as reflected in the phases of the South 033257 Bowen Airport –20.02 148.22 Oscillation Index (SOI), a convenient and robust measure of 033007 Bowen Post Office –20.02 148.25 categorising ENSO states (Nicholls et al. 1996; Stone et al. 033094 Bowen Qld Salt –20.01 148.23 1996). Scherm and Yang (1995, 1998) reported ENSO- 035019 Clermont Post Office –22.83 147.64 related disease outbreaks and severity for wheat rust in China 041522 Dalby Airport –27.17 151.27 and the United States. 041023 Dalby Post Office –27.18 151.26 In this paper we report on the predictability of climate 065012 Dubbo (Cooreena Rd) –32.21 148.57 variability effects on the potential number of sorghum ergot 065037 Dubbo State Forest –32.27 148.62 events during the year. The analysis was conducted based on 035264 Emerald Airport –23.57 148.17 035027 Emerald Post Office –23.53 148.16 the findings of Wang et al. (2000a, 2000b) using phases of 040082 Gatton Lawes College –27.55 152.34 the SOI (Stone et al. 1996). 040436 Gatton QDPI Research Stn –27.55 152.33 041521 Goondiwindi Airport –28.52 150.33 Materials and methods 041038 Goondiwindi Post Office –28.55 150.31 075041 Griffith Aero –34.26 146.06 Historical weather data and estimation of daily wetness 075028 Griffith CSIRO –34.32 146.07 Long-term historical climatic data (1 January 1887–30 May 1998) from 073128 Gundagai Ridge Street –35.08 148.1 locations throughout the sorghum-growing areas of Australia were 055023 Gunnedah Pool –30.99 150.25 obtained from SILO patch-point data-set (Table 1). There are no recorded humidity or vapour pressure data available; hence, daily 055024 Gunnedah SCS –31.03 150.27 075018 Hay Corrong –34.22 144.46 maximum vapour pressure deficit (VPDmax) was estimated to replace the relative humidity factor in the model developed by Wang et al. 075134 Hay Epsom Downs –34.88 145.29 (2000b). VPDmax was estimated from daily minimum and maximum 075031 Hay Miller Street –34.52 144.85 temperature by assuming that the daily minimum temperature equals 040112 Kingaroy Prince St –26.55 151.85 dew point and that dew point does not change during any single day 002038 Kununurra –15.78 128.74 (Tanner and Sinclair 1983). Meinke et al. (1997) demonstrated the 002056 Kununurra Aero –15.78 128.71 validity of this approach for south-eastern Queensland: 040083 Gatton Post Office –27.56 152.28 VPDmax = ETmax – ETmin (hPa) 033045 Mackay Aero –21.17 149.18 031066 Mareeba QWRC –17.00 145.43 ETmax = 6.107 exp[17.4 Tmax /(239 + Tmax)] (hPa) 042023 Miles Post Office –26.66 150.18 ETmin = 6.107 exp[17.4 Tmin /(239 + Tmin)] (hPa) 039104 Monto Post Office –24.87 151.12 where VPDmax is the maximum daily vapour pressure deficit, and ETmax 012056 Moree –31.6 119.14 and ETmin are the saturation vapour pressure at daily maximum 053048 Moree Comparison –29.48 149.84 temperature (Tmax) and minimum temperature (Tmin), respectively. 053027 Moree Post Office –29.5 149.9 054120 Narrabri Bowling Club –30.32 149.79 Simulation of the critical growth stages 053030 Narrabri West Post Office –30.34 149.75 For each day of the year, assumed as the first day (beginning) of 051115 Narromine Airport –32.22 148.23 flowering, the duration of flowering period, the starting date, and the 051037 Narromine Post Office –32.23 148.24 length of the flag leaf period were predicted. The starting date of the flag leaf stage was simulated by assuming that the flag leaf begins to 002041 Ord River Regeneration St –17.39 128.92 emerge when the third leaf prior to the flag leaf is fully expanded 041082 Pittsworth Post Office –27.71 151.63 (G. Hammer, pers. comm.), and that each of the last 3 leaves requires 039083 Rockhampton Aero –23.38 150.48 20 degree-days to fully expand. The end of the flag leaf stage was 043091 Roma Airport –26.55 148.78 simulated using the APSIM sorghum model (Hammer and Muchow 043030 Roma Post Office –26.57 148.79 1994). Anthesis was assumed to begin 100 degree-days after the flag 035065 Springsure Post Office –24.12 148.09 leaf had fully expanded, and the length of the flowering period was 043034 St George Post Office –28.04 148.58 calculated using a thermal time value of 128 degree-days with a base temperature of 3.2°C. These assumptions were based on the original 041103 Toowoomba –27.58 151.93 data of Hammer et al.