Effects of timing of Leptosphaeria maculans ascospore release and fungicide regime on phoma leaf spot and phoma stem canker development on winter oilseed rape (Brassica napus) in southern England

Abstract

winter oilseed rape growing seasons, numbers of air-borne ascospores of Leptosphaeria maculans were often > 4 m − 3 from autumn (September/October) to spring (April / May), while few or no ascospores were detected during the summer. Mature pseudothecia were generally not observed on debris of the previous crop until September. One-year-old debris (harvested in July 1998) had 95% discharged and 5% mature pseudothecia in August 1999, but by 15 September new pseudothecia (of which 30% were mature) were observed and the first increase in air-borne ascospores (> 4 m − 3 ) occurred. Phoma leaf spotting appeared in untreated field plots 14 -25 days after the first increase in air-borne ascospores in autumn. The fungicide mixture difenoconazole plus carbendazim decreased the incidence of new leaf lesions for 1 month after application in autumn and for 2 months in midwinter. When L. maculans was isolated from infected leaves, the growth rate of isolates from leaves to which fungicide was applied was less than that of those from untreated leaves. Foliar applications of fungicide to field plots in the autumn and winter not only decreased the incidence of crown cankers but also reduced the rate of canker development on stem bases in the spring and early summer (when severity of crown cankers increased linearly with time). In untreated crops, when phoma leaf spots appeared early in the autumn, crown cankers developed early in the spring but only became severe enough before harvest to reduce yield greatly in 1997/98. Yield loss was associated with crown cankers that girdled more than half of the stem by harvest (mean severity > 3 on a 0 -5 scale). Infections of new leaves produced after stem extension, from January onwards, led to phoma stem lesion development above the crown. In the three seasons, phoma stem lesions became moderately severe (> 2) by harvest only in untreated plots in 1997/98.

Susceptibility of winter rape cultivars to fungal diseases and their response to fungicide application

“…Chemical control of phoma basal canker is possible only by fungicide application in the autumn. Disease incidence on winter rape leaves in autumn (usually 10%-20% of plants with phoma leaf spots) is an indicator for fungicide application against phoma stem canker (West et al, 2002;Gladders et al, 2006;Steed et al, 2007). Despite the fact that during our experimental period fungicide was applied in autumn at the optimal time (when the incidence of phoma leaf spot ranged from 10% to 25%), the effect of the autumn fungicide spray application on the incidence and severity of basal stem canker was not always meaningful.…”

“…Effective control of phoma stem canker can also be achieved by foliar fungicide sprays in the autumn; however, optimal timing of such applications is very important (West et al, 2002;Aubertot et al, 2006;Gladders et al, 2006). Useful effects may also be achieved through the choice of resistant cultivars (Delourme et al, 2006;Fitt et al, 2006a).…”

Advances of Fungicide Application for Winter Oilseed Rape

“…Infection by ascospores is possible during all autumn period and also during the succeeding spring and summer under conditions of Latvia. Infection of leaves was low in autumn, and it might be possible that infection of stems does not occur in autumn like it is in countries with a milder climate (West et al, 2002). Leaf spot most frequently was observed on the oldest leaves which died during winter, and therefore did not influence development of stem canker.…”

Monitoring blackleg (Leptosphaeria spp.) ascospore release timing and quantity enables optimal fungicide application to improved oilseed rape yield and seed quality

“…Wind-borne ascospores adhere to the leaf surface and germinate to produce germ tubes, which penetrate the leaf indirectly through stomata or wounds; endophytic mycelia of the pathogens then grow along the leaf petioles asymptomatically to cause lesions on main and branch stems (Hammond et al 1985). Monitoring of the presence and concentration of fungal spores in air samples makes it possible to identify periods of disease risk to plants (West et al 2002a;Piliponyte-Dzikiene et al 2014). The detection of airborne inoculum sources at early stages of plant infection allows to assist in decisions on the optimal time for fungicide application (Gladders et al 1998).…”

“…There are many studies connected with the effectiveness of fungicides in relation to various fungal diseases (West et al 2002a;Brazauskiene and Petraitiene 2004;Steed et al 2007;Karolewski et al 2009), but knowledge regarding their influence on plant physiology and quality of harvested seeds is poor. Most studies focused on the impact of single applications of fungicides on yield, but they have not examined in detail the relationship between fungicide treatment and plant health, seed yield and their quality in relation to spore concentration in the air.…”