29Ascochyta rabiei asexual spores (conidia) were assumed to spread over short distances 30 (~10 m) in a combination of rain and strong wind. We investigated the potential distance of 31 conidial spread in three rainfall and three sprinkler irrigation events. Chickpea trap plants 32 were distributed at the distances of 0, 10, 25, 50 and 75 m from infected chickpea plots 33 before scheduled irrigation and forecast rainfall events. Trap plants were transferred to a 34 controlled temperature room (20 °C) for 48 h (100% humidity) after being exposed in the field 35 for 2-6 days for rainfall events, and for one day for irrigation events. After a 48 h incubation 36 period, trap plants were transferred to a glasshouse (20 °C) to allow lesion development.
37Lesions on all plant parts were counted after two weeks, which gave an estimate of the 38 number of conidia released and the distance travelled. Trap plants at all distances were 39 infected in all sprinkler irrigation and rainfall events. The highest number of lesions on trap 40 plants were recorded closest to the infected plots -the numbers decreased as the distance 41 from the infected plots increased. There was a positive relationship between the amount of 42 rainfall and the number of lesions recorded. A generalised additive model was developed that 43 efficiently described spatial patterns of conidial spread. With further development, the model 44 can be used to predict the spread of A. rabiei. This is the first systematic study to show that 45 conidia distribute A. rabiei over longer distances than previously reported. 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 Keywords: Ascochyta blight, epidemiology, conidial spread, wind-driven rain, chickpea, 62 sprinkler irrigation.63 Introduc)on 64 Chickpea (Cicer arietinum L.) is the second most important legume crop globally and is the 65 most widely grown legume grain crop in Australia with >1,060,000 ha harvested in 2018 66 (FAOSTAT 2020). Ascochyta blight caused by Ascochyta rabiei (syn. Phoma rabiei) is one of 67 the most devastating chickpea diseases worldwide (Pande et al. 2005). With the exception of 68 the Ord region in northern Western Australia, A. rabiei is the major biotic constraint to 69 chickpea production in Australia with almost all chickpea growing areas affected (Bretag et 70 al. 2008). Ascochyta rabiei survives on infected seed, volunteer chickpea plants and infested 71 stubble, forming conidia that initiate primary infection. Infected seed gives rise to infected 72 seedlings through transmission from germinating seeds. Conidia are spread by rain splash or 73 wind driven rain. Pycnidial formation, conidial production, host infection and disease 74 development are favoured by temperatures between 5 and 30 °C (optimum 20 °C), relative 75 humidity > 95 % (Nene 1982), and wetness period of 10 h or more (Khan 1999). A longer 76 wetness period is required for spore germination at sub-optimal temperatures, and infection 77 is rare in hot and dry conditions (Jhorar et al. 1998). Symptoms develop within 5...