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Light leaf spot alert for oilseed rape in the United Kingdom

A ProMED-mail post <http://www.promedmail.org>
ProMED-mail is a program of the International Society for Infectious Diseases <http://www.isid.org>

Date: Mon 19 Jan 2015
Source: Farmers Weekly [edited]
<http://www.fwi.co.uk/arable/light-leaf-spot-red-alert-for-oilseed-rape-growers.htm>

Light leaf spot red alert for oilseed rape

Light leaf spot levels are building to alarming levels in oilseed rape crops. The problem is becoming ever more acute each year across the UK -- not just in Scotland and northern England, where the disease has traditionally been a headache.

With few varieties having solid resistance, crops are vulnerable.

According to Crop Monitor's preliminary 2015 forecast, almost all of the UK is at very high risk due to last [2014] season's high infection levels resulting in carryover of inoculum.

Julie Smith, ADAS [Agricultural Development and Advisory Service], warned symptoms are now being seen across UK sites. The Crop Monitor risk alert is based on the percentage of crops with more than 25 percent plants infected. "[But] I think we are underestimating the problem. If there is 15 percent infection at stem extension, you can expect a yield loss of 5 percent."

Light leaf spot is polycyclic, meaning several infection cycles can occur throughout the season, so ensuring adequate protection is crucial. It is also difficult to detect in the early stages and even if obvious symptoms can't be seen, the disease may still be present.

Ms Smith said there is no treatment threshold, as soon as the disease is detected, a fungicide should be applied. "For those concerned about the cost of an extra spray, remember that the disease can cause yield losses of up to 1t/ha," she added.

Light leaf spot isolates with reduced sensitivity to azole fungicides have been identified, but there is no evidence this is having a significant effect in the field. However, Ms Smith said it is a major concern that there are so many azole fungicides being applied. "You are selecting for resistant strains," she added. There are now alternative products on the market, [that] could be used either side of an azole-based spray to bring in some different modes of action.

[Byline: Adam Clarke]

Communicated by:

ProMED-mail

<promed@promedmail.org>

[Light leaf spot (LLS) of _Brassica_ species is caused by the fungus _Pyrenopeziza brassicae_. In oilseed rape (_Brassica napus_), leaves become infected soon after sowing, but remain symptomless until lesions appear on older leaves. Developing pods are often infected, leading to premature ripening of seed and pod shatter. LLS is polycyclic and survives the summer on crop debris, volunteer oilseed rape, and vegetable brassicas. Disease management may include varieties with decreased levels of susceptibility, disease monitoring, and timely fungicide applications.

In the UK, LLS has caused considerable yield losses of oilseed rape in the past estimated at GBP 40 million (about USD 60 million) per year.

For many fungal crop diseases, new pathogens strains with increased virulence and fungicide resistances are emerging. Thus, pathogen resistance is of major concern to primary industries worldwide.

Rotating or mixing chemical classes of fungicides, as suggested above, will aid in extending the useful life of the various compounds.

Maps

UK:

<http://www.lib.utexas.edu/maps/europe/united_kingdom.gif> (with

counties) and

<http://healthmap.org/promed/p/40>

<http://www.picturesofengland.com/images/mapofengland/england-counties.gif>

Pictures

LLS symptoms on oilseed rape

- leaves:

<http://www.farmersguardian.com/Pictures/inline/d/t/n/AF_leaf_spot.jpg>,

<http://www.farmersguardian.com/pictures/610xAny/4/3/2/35432_p22_pic1_Feb_26.jpg>,