Oilseed rape diseases

The review considers the diseases of oilseed rape in the context of UK Agriculture. The recent and relevant literature, together with the authors experienced observations and results of discussions with plant pathologists involved in research and extension, are reviewed and discussed.

Three principle diseases have been identified as posing the main threat to the UK crop, namely light leaf spot (Pyrenopeziza brassicae), dark leaf and pod spot (Alternaria brassicae and A. brassicicola) and canker (Leptosphaeria maculans). Diseases which are common, but are considered to be less damaging are beet western yellows virus (BWYV), sclerotinia stem rot (Sclerotinia sclerotiorum) and grey mould (Botrytis cinerea). Other diseases, cauliflower mosaic virus (CaMV), turnip mosaic virus (TuMV), club root, (Plasmodiophora brassicae), damping-off caused by pythium spp. and Rhizoctonia solani, phytophthora root rot (Phytophthora megasperma), powdery mildew (Erysiphe cruciferarum), downy mildew (Peronospora parasitica), white leaf spot (Pseudocercosporella capsellae) and ringspot (Mycosphaerella brassicicola) are considered to be of relatively minor importance nationally. Little published data exists on some of these diseases and they are reviewed in less detail. The existence of white blister (Albugo candida} and verticillium wilt (Verticillium dahliae) has not yet been confirmed in the UK, but because of their potential for affecting yield of oilseed rape, they are covered in this Review.

The data reviewed show that disease incidence and yield response generally decline as crops are grown further north in England, but increase again in Scotland. The best estimates of yield losses from any disease comes from extrapolation from early work on dark leaf and pod spot, where for every one per cent disease on the pods it was calculated that there was a one per cent loss in yield. When this formula was applied to disease severity as recorded in the ADAS disease surveys for England and Wales for the years 1986-1990, the annual estimated loss due to disease was about five per cent. For an average crop yield of 3.05 t/ha at £240/t for double low cultivars this would be equivalent to £36/ha. National losses due to disease would be in the order of £12M per year.

In calculating the economics of disease control by using fungicides, it was estimated that the cost of a single fungicide application was equivalent to 0.1 t/ha. For a two-spray programme, including an application following the onset of flowering, 0.3 t/ha would be required, as it is necessary to take into account wheeling losses from the later application. For the average crop this is equivalent to a yield response of nine per cent (current proposals by the European Commission to alter the support system for oilseed rape could radically alter the economics of fungicide treatment for disease control).

Light leaf spot is now considered to be the main fungal disease of oilseed rape in the UK. Disease/yield loss relationships are uncertain, but control can be achieved by fungicides applied in the autumn and early spring. There is resistance to the disease in many current cultivars but, when disease pressures are high, reliance has to be placed on fungicides for control.

Leptosphaeria maculans, the cause of leaf spot and canker, exists in two distinct strains. Systemic infection by the fungus has been identified as a possible problem for disease control with the available protectant fungicides. Sources of genetic resistance are available, but current cultivars generally have low levels of resistance and are potentially at risk.

Alternaria spp. infecting oilseed rape are of worldwide distribution and dark leaf and pod spot is of importance wherever oilseed rape is grown. Yield losses due to the disease can be high and the lack of genetic resistance in all current commercially available cultivars make the disease a potential problem in the UK. Control of the disease is heavily dependent on fungicides. There is a major requirement to develop an effective forecasting system to predict high risk conditions which lead to epidemics of the disease.

Sclerotinia stem rot is widely recognised as an important disease, yet evidence that it causes major losses is limited. It is only of consequence in the UK in about one year in ten and the major outbreaks occur in a limited geographical area in Sussex and Kent. 1991 was considered to be a bad year for sclerotinia. It is possible that outbreaks may be more frequent in future as inoculum levels increase. Experimental evidence suggests that for control measures to be economic a disease incidence of more than 25 per cent of plants infected is required. There are currently no cultivars resistant to the disease and control is achieved by fungicides applied at a time which coincides with ascospore release during the flowering period. A number of forecasting schemes have been devised, including monitoring of depots of buried sclerotia, to predict high risk conditions. These schemes met with limited success under UK conditions, but there is further scope for their evaluation.

There is little published data on grey mould on oilseed rape. The fungus is considered to be an opportunist, colonising senescing tissues or tissues that have been damaged by weather, pest, the passage of machinery or fertiliser scorch. The disease can be damaging under wet conditions. Although stem infections can create the appearance of widespread infection in crops as plants ripen, specific control measures are not considered to be economic for this disease.

Beet western yellows virus has been found to be ubiquitous and, surprisingly, more common in the north of England in recent years. The disease is almost symptomless and yield loss has been difficult to evaluate. As the rape host constitutes a major reservoir of the virus the threat from the virus to other crops, such as sugar beet and lettuce, requires evaluation. Of recent concern is the dramatic increase in the recorded incidence of the cauliflower and turnip mosaic viruses, recorded on oilseed rape in some parts of the country for the first time in 1991. These two viruses can be severely damaging and their future development in oilseed rape will require close monitoring.

Of the other diseases covered by the Review, the relatively few serious outbreaks of club root are surprising considering the susceptibility of oilseed rape to the disease. Downy mildew, though widespread and causing leaf loss in mild wet autumns and early springs, is not considered to be a major cause of either crop failure or yield loss. Premature senescence of plants within cops has caused concern since the early 1980s. Rhizoctonia has been implicated, but virtually no research into this problem has been undertaken in the UK. Powdery mildew was common in the dry year of 1990, but has not been a problem in 1991, and is likely to remain a sporadic disease. White leaf spot and ring spot are primarily confined to the South West of England and Wales and no general outbreaks have been reported.

The effects of disease on oil quality and glucosinolate content have been evaluated. There was some evidence that control of disease by fungicides can improve oil content and reduce amounts of glucosinolates. However, it was sometimes difficult to separate the effects of the fungicide treatment on diseases from those on crop maturity which also has an influence on glucosinolate content.

Six areas are identified for further research and development: surveys, disease/yield loss relationships, epidemiology, forecasting and thresholds, fungicide spray timing and comparisons and host genetic resistance. The major area identified for immediate work is that of establishing diseases/yield loss relationships for the main pathogens and the forecasting of disease risk.

The Review includes 250 key references covering all the major aspects of the symptoms, aetiology, epidemiology and control of the diseases of oilseed rape.