Date: 05/2022

Earlier drilling has definitely helped avoid the worst of the UK’s autumn flea beetle problems, but growers need to be aware of the implications to Light Leaf Spot and manage crops accordingly, says DSV technical manager Andrew Hartley.

As is the case in many agricultural situations, if you successfully manage one area of concern, it can upset another and bring other problems to the fore.

Light Leaf spot (LLS) has long been a disease more prevalent in the North of the country and resistant varieties have done much to keep incidences at bay in recent years, but there’s growing evidence that the condition is becoming more of a problem further south now.

It’s easy to see why. The sooner you drill oilseed rape in the autumn, the more plants are able to make use of residual soil warmth, relatively long days and generally better growing conditions than later in the season.

Such a situation benefits rapid establishment and helps the plant grow through early pest and disease attack which is why it has been such a useful strategy in addressing cabbage stem flea beetle problems in many parts of the country.

The trouble is such conditions also favour the development of other things that like to grow in warmer conditions, including plant pathogens, particularly fungi and moulds. Generally speaking the earlier you drill in the autumn, the more susceptible crops are to diseases.

In the case of LLS, leaving the previous season’s OSR volunteers longer before destroying, again to offset flea beetle, has increased the risk factors whilst the warmer, wetter autumns and winters we are now seeing as a result of climate change are playing their part, too.

Add to this a disease with more than one infection cycle a year and the variable nature of the incidence of the disease from one region to another and even field to field, and you can appreciate why it needs careful monitoring.

Widespread incidence

Travelling recently around trials from Scotland to the south of England, LLS can be found in most locations at variable levels in both untreated and treated plots.

Although it is notoriously difficult to identify particularly at low levels, it shows itself around this time of year when autumn treatment has lost its efficacy and symptoms begin to show particularly on new growth.

Fast growing OSR can leave LLS infection in the bottom of the crop so on initial inspection the crop looks clean only for disease to move up the plant if conditions are right.

Visual symptoms vary from the classic cream centered lesion with dots like salt grains around the outside to distorted leaves, brown brittle edges and stunting, later infection can be found on stems and pods which is likely to lead to pod shatter.

Yield losses can be up to 30% from spring infection and yield responses to control can average between 0.3 to 0.6t/ha so it’s important to put a number of measures in place to minimise risk.

A good starting point is variety choice. On the 2022/23 AHDB RL, the DSV varieties Matrix CL, Darling, Dazzler and Crocodile all feature 6s for LLS resistance whilst DSV Dart has a 7. Our new variety DSV Daktari also has a 6.

In many cases this is combined to strong resistance to other common disease such as phoma stem canker, TuYV and verticillium wilt, making them exceptionally resilient varieties.

High quality seed critical

With LLS, it’s always a good starting point to choose high quality seed and combine this with the destruction of any potentially infected crop debris from previous OSR crops. This prevents a residue of infected spores and establishes a good healthy fast growing crop.

Usually the autumn fungicide application to control phoma controls LLS pre-Christmas and a preventative strategy for LLS is the best approach rather than reacting to visual symptoms.

However if LLS can be found in crops prior to, or at stem extension, the advice would be to spray promptly as there are no thresholds.

There is a good choice of products but some concerns over LLS insensitivity to triazoles, so as with all application decisions, use active ingredients from other groups in mixtures and switch products where possible in a program.

Although LLS is definitely on the rise across the UK, it can easily be kept in check by correct varietal choice and sound management.