EFFECT OF SCLEROTINIA SCLEROTIORUM INOCULUM, WEATHER CONDITIONS, CULTIVARS, PLANT DENSITIES AND FUNGICIDE TIMING ON STEM ROT AND CROP YIELD OF OILSEED RAPE
JMLI DAVIES1, CS YOUNG2, P GLADDERS3, LC HIRONl, JA SMITH2and M WATLING1
1ADAS Terrington, Terrington St Clement, Kings Lynn, Norfolk PE34 4PW, UK; 2ADAS Wolverhampton, Woodthorne, Wergs Road, Wolverhampton WV6 8TQ, UK; 3ADAS Boxworth, Boxworth, Cambridge CB3 8NN, UK
Background and objectives
Effective control of stem rot can be achieved by fungicidal application during flowering but current ADAS work has shown that 66% of fields are treated unnecessarily. More work is required in identifying the crop risk by the development and appraisal of a rapid petal test supported by a critical weather forecasting system. Cultural practices may play a part in minimizing disease, as reported from Canada where more stem rot was reported at higher plant densities . The objective was to obtain fundamental information on the relationship between canopy height, crop density, petal infection by S. ;sclerotiorum, petal retention, weather conditions during flowering and fungicide timing on final disease levels and crop yield of winter oilseed rape.
Materials and methods
Three fully replicated plot experiments, designed as a three-way factorial randomized block, were established on cvs. Rocket and Apex grown in high risk sclerotinia affected sites at Crowland, Lincs., in 1995-96 and in 1996-97 and at Severn Stoke, Worcs., in 1996-97 at three plant densities (mean densities of 43, 72, and 97 plants/m2 in the spring). Iprodione plus thiophanate-methyl was applied at full rate at early, mid and late flowering. Plant densities were recorded in the spring. Crop growth stage, the numbers of leaves on the main stem of tagged plants and the number of petals adhering to each leaf and to each leaf axil were recorded weekly during flowering. Scierotial depots were established and assessed weekly for the presence of apothecia. Petals were collected from untreated plots and cultured onto PDA amended with streptomycin. The plates were incubated at 20°C and assessed after 10-12 ;days for the presence ofS. ;sclerotiorum. The incidence and height of stem rot lesions was assessed.
Results and conclusions
'Rocket' retained on average one more leaf per plant than 'Apex', and retained more petals on the leaf blades. Apex retained more petals in the leaf axils at the both sites in 1997. No appreciable differences in plant height were observed, despite 'Rocket being treated with growth regulator. At the lower plant densities, more leaves were retained per plant and more petals were retained on leaf blades and in leaf axils at two sites (Crowland 1996 and Severn Stoke 1997). In 1997, for the first time, at both sites a high mortality of buried sclerotia occurred over winter. The start of sclerotial germination in depots did not consistently coincide with initial petal infection, as determined by petal testing. Petal testing was, therefore, considered more reliable. At Crowland in 1997, the mean petal infection at late flowering was 64%, confirming the site was still high risk 6 ;years after the first severe disease in 1991. Stem rot at harvest was at trace levels in 1996. In 1997, an overall mean of 11% plants affected was recorded in the untreated plots at both sites. Although fungicide treatment reduced stem rot, there were no differences between fungicide timings and cultivars and no effect on crop yield. The highest yields were recorded from the lower plant densities (mean yields of the three sites of 5.06, 4.87 and 4.47 t/ha at low, moderate and high densities, respectively). Although at a low level, more stem rot was reported at higher plant densities at Crowland in 1997. Examination of the 1997 data showed that petal deposition within the leaf layers did not appreciably differ between low and high plant densities, thus supporting the Canadian work  which showed that the effect of the higher plant density on stem rot may be that of a more favourable microclimate. There was sufficient moisture on the leaves for petal retention but the lack of moisture after petal fall was considered responsible for the low levels of stem infection at most sites. Low temperatures may have also limited host infection at Crowland in 1997.
1. Turkington TK, Morrall RAA, Phytopathology 83, 682-689