Neil Groom, technical director for Grainseed, is of the view that we need to utilise the variety disease ratings in oilseed rape much more than we do. “In wheat most growers would sit down with the AHDB variety list in front of them and make their variety selection based on a number of factors –yield usually comes first, followed by disease ratings. The intrinsic ability of the wheat variety to withstand Septoria, for example, would be high on this list of most growers’ needs. So in rape the variety’s intrinsic ability to withstand Phoma, which is the most common and the most damaging disease of rape, should be high on the list of requirements. Phoma stem canker can erode yields by at least 0.5 to 1 t/ha, which is worth a lot of money in rape and can mean the difference between a profitable crop and an unprofitable one. It remains the most significant and most costly disease in rape.
According to the Crop Monitor surveys, Phoma remains the most serious and widespread disease in rape, affecting 82% of crops and 21% of plants in spring 2019 and 89% of crops and 40% of plants in autumn 2018. In the Southeast, 100% of crops had Phoma. It is much more widespread than Light Leaf spot, which in spring 2019 infected 64% of crops and 23% of plants and in autumn 2018 infected just 3% of crops and 0.2% of plants. “You would have expected more Light Leaf spot following the mild winter last year, but that didn’t appear to be the case. I think we over play Light leaf spot and under play Phoma. The use of Phoma resistant varieties may not obviate the need for fungicide treatments altogether, but it will allow more flexible timings and possible fewer fungicide applications and a robust start which is all important for rape. An integrated approach to disease makes economic and agronomic good sense.”
“Most Grainseed varieties have strong Phoma resistance rating and this genetic resistance to Phoma is multi-gene which is more durable. They also have excellent tolerance to Verticillium where there are still no chemical
solutions to this disease. Es Mambo has a 7.8 rating for Phoma stem canker, it is a top performing variety in AICC Verticillium trials. Keeper has a 7 rating for Phoma and it too shows resistance to Verticillium wilt. Decibel CL has an 8 for Phoma.”
Usually it is around now that you start to see the first signs of stem canker caused by Phoma are visible at the stem base on susceptible varieties or the first signs of Verticillium. Growers are advised to remove individual plants and scrape the surface around the stem base to confirm diagnosis. Sunken brown or necrotic lesions at the stem base may indicate stem canker. These lesions can encircle the stem and weaken it, causing premature ripening, lodging or even plant death.
If you suspect Verticillium, symptoms are usually seen a little later, end of June or early July. For Verticillium, look for yellowing leaves, premature ripening of branches and grey stripes down stems, often on one side only, or if severe the whole cross-section of a stem. Underneath these vertical stripes, when the outer stem layer is peeled off, you will see grey vascular tissue and using a hand lens, you can see tiny black dots or microsclerotia.
Mr Groom reminds us that the most important source of Phoma infection is airborne spores from oilseed rape stubble after harvest. “At least there is less rape stubble around this season. We need to monitor for twenty days of rain from the 1st of August to trigger the release of air-borne Phoma spores which then infect newly-emerged oilseed rape plants to produce leaf spots. These infecting ascospores can be blown some distances but are generally deposited within 300 metres. The most damaging effects of Phoma are found in smaller plants in susceptible varieties. This is because the fungus has to grow a shorter distance down the leaf petiole to reach the stem base where it forms cankers right around the stems.”
Mr Groom says “controlling Phoma needs careful management including the use of rape varieties with ratings of 7 or above, cultivations, rotations and suitable fungicides.”