Pea yield decline syndrome in South Australia: the role of diseases and the impact of agronomic practices

Sixty commercial pea crops were surveyed in 1995 to determine the causes of declining yields. Blackspot (Mycosphaerella pinodes and Phoma medicaginis var. pinodella) and downy mildew (Peronospora viciae) were prevalent in most crops and were identified as probable major contributors to the syndrome. Short rotation intervals (<5 years) between pea crops in paddocks were correlated with increased levels of blackspot and lower grain yields. Early sowing dates were correlated with increased levels of blackspot. A detailed survey of blackspot development was conducted in 5 commercial paddocks in 1996. The relative importance of sowing time and rotation varied between regions and seasons. The impact of a range of herbicides and the micronutrients manganese and zinc on blackspot, caused by the M. pinodes component of the blackspot complex, was investigated in a field trial during 1996. The herbicides diuron, metribuzin, and fluazifop significantly increased blackspot crown lesions compared with the nil treatment. There was a significant interaction between blackspot severity, herbicides, and the micronutrients manganese and zinc. Manganese concentration in pea plants was negatively correlated with the severity of blackspot crown lesions and positively correlated with severity of downy mildew.