1ARC-Grain Crops Institute, Private Bag X1251, Potchefstroom, 2520 Republic of South Africa; 2Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria, 0002, Republic of South Africa

Background and objectives
Stenocarpella maydis ear rot results in yield losses, grain quality reductions and animal toxicity in maize grown worldwide. The pathogen survives on host residues. Previous studies have shown that minimum tillage and concomitant increases in surface residues can promote the disease. Advantages of minimum tillage have resulted in farmers being reluctant to revert to conventional tillage to reduce S. maydis ear rot incidence. The aim of this study was to determine the efficacy of alternating reduced and conventional tillage systems for reducing input costs and inoculum, thereby managing maize economically.

Materials and methods
A field trial was carried out at Bloekomspruit (South Africa), where high natural infections of S. maydis ear rot occur, from 1993-94 to 1995-96. Tillage systems included rip-on-row, mouldboard plough, shallow chisel, no-tillage and a disc-plough treatment. Tillage treatments were interrupted by a mouldboard cross-plough treatment after 1, 2 and 3 years, respectively. Plot sizes were 9x25 m with 1.5 m inter- and 0.3 m intra-row spacings. Treatments were replicated three times in a randomized complete block design. S. maydis rotten ears (%), visually rotten disease kernels (% w/w) and S. maydis isolation frequency (%) were used to quantify disease incidence. Stubble mass and pycnidia/cm2 were also quantified.

Results and conclusions
Mouldboard ploughed plots had consistently lower stubble mass and consequently reduced Stenocarpella ear rot incidence and severity more than reduced tillage systems. A cross-plough treatment applied after one, two and three seasons of reduced tillage reduced stubble mass and Stenocarpella ear rot incidence and severity in the respective season only. Stenocarpella ear rot incidence increased during the following season in which the original tillage treatments were again applied. A linear relationship between stubble mass and S. maydis ear rot incidence was recorded. Surface stubble accounted for 46.90, 54.40 and 73.15% of the variation in S. maydis ear rot incidence during the 1993-94, 1994-95 and 1995-96 seasons, respectively. Results imply that a single season mouldboard plough decreased Stenocarpella ear rot incidence by reducing stubble mass for the particular season only. Alternating tillage systems would, therefore, not reduce Stenocarpella ear rot in the long term, but continuous mouldboard plough significantly reduced Stenocarpella ear rot incidence.