Effects of fungicide application timing on the incidence and severity of Alternaria blight (Alternaria brassicae) and on the productivity of spring oilseed rape (Brassica napus L. ssp. oleifera annua Metzg.)

I. Brazauskiene and E. Petraitiene

Lithuanian Institute of Agriculture, Department of Plant Pathology and Protection, Instituto al. 1, Akademija, Dotnuva, LT-58344, Kedainiai distr., Lithuania;
e-mail: brazausk@lzi.lt, egle@lzi.lt

Abstract:

Three field experiments were carried out with the spring oilseed rape (Brassica napus L. ssp. oleifera annua Metzg.) cv. ‘Star’ to investigate the incidence, severity and harmfulness of Alternaria blight (Alternaria brassicae) and to test the possibility of reducing the disease pressure by fungicidal spray applications of 45% Sportak (a.i. prochloraz) 0.675 kg a.i. ha^-1 and 25% Folicur (a.i. tebuconazole) 0.25 kg a.i. ha^-1. The fungicides were applied at different times, i.e. after the first spots ofAlternaria blight had appeared on the lower, middle and upper leaves or on siliques and at the end of spring oilseed rape flowering. Alternaria blight was present in crops of the spring oilseed rape cv. ‘Star’ in all the experimental years. The disease severity varied in individual years and was heavily dependent on the weather conditions (amount of precipitation and temperature). Of all the experimental years, the most conductive conditions to the spread and development of Alternaria blight on spring rape siliques occurred in 1998, when disease spots covered 18.65% of the surface area of siliques in the untreated plots. The tested fungicides had little effect on the disease incidence, however, prochloraz and tebuconazole applied on all dates declined the disease severity. The highest efficacy was recorded when the fungicides were applied after the first symptoms of Alternaria blight had been spotted on siliques. Tebuconazole suppressed the disease severity more effectively than prochloraz.
In the year most favourable for Alternaria blight occurrence (1998), the seed yield in the untreated plots was by up to 0.07 t ha^-1 lower, and the disease severity on siliques was as much as 3.2 times higher than in the fungicide-sprayed treatment. The highest average spring rape seed yield increase resulting from fungicidal spray applications during the period 1997–1999 amounted to 0.040 t ha^-1. Fungicides declined the content of Alternaria blight diseased seeds per silique, increased 1,000-seed weight, however, no significant effect of fungicides was identified on the number of siliques per plant and the number of seeds per siliqua.

Key words:

Alternaria brassicae, disease incidence, disease severity, oilseed rape, prochloraz, seed yield, tebuconazole