Phoma leaf spot risk is now high
30 Oct 2018
Phoma leaf spot risk is now high and the first leaf spot symptoms have been reported in areas of England on mid– August sown crops.
Here, Tim Boor, Plant Pathologist, ADAS and Adrian Cottey, Head of seeds Northern Europe, BASF talk about managing the disease in the current season.
Tim said, “The rule of thumb for ascospore (airborne spores) release from infected stubble is around 20 days of rain since the 1st August and this was reached on the 10th of October in Cambridgeshire.
Normally we would expect it to take a couple of weeks for these spores to produce visible lesions.
Proximity to a previous oilseed rape crop
Generally, the closer the crop is to the previous oilseed rape crop, the greater the risk, especially if those stubbles have not been incorporated as ascospore release comes from infected stubbles.
In terms of location, the disease is likely to have the biggest impact in eastern, southern and central areas of England.
Phoma leaf spot and stem canker is one of the most important diseases of winter oilseed rape in England and in a high-risk year can account for losses of 0.5-0.7t/ha.
Smaller plants are at the greatest risk from phoma, so later sown crops are usually more at risk. This year, later sown crops suffering from the lack of rain during September and early October are likely to be small.
The fungus grows down the leaf petiole towards the stem. In smaller plants, this distance is shorter and so phoma infection can reach the stem base quicker.
In larger plants it takes longer for phoma to reach the stem, and in some cases, affected leaves drop off before this happens, reducing risk.”
Adrian said, “The varieties with the highest stem canker resistance scores, e.g. 8 or 9, will usually have major gene resistance. As a result, they have low levels of stem canker at harvest and tend to have low levels of leaf spotting in the autumn and early spring. Normally, varieties with specific resistance genes also have a background level of resistance as well, coming from a number of quantitative genes.
Other varieties have a varying level of resistance from the quantitative genes, and although this can result in relatively good stem canker scores e.g. 6 or 7, it is more usual to see higher levels of phoma leaf spot in such varieties in the autumn.
Varieties with stem canker scores of 3, 4 or 5 are much more susceptible to both the stem canker and leaf spot phases of the disease and it is these varieties that need to be carefully monitored for exceedances of the threshold for phoma leaf spot.”
Tim said, “Weather data is used to predict when the 10% incidence of phoma leaf spotting is likely to be reached and so aids in planning fungicide applications. The forecast model is available from https://ahdb.org.uk/knowledge-library/leafspots. It suggests that thresholds will be met end of October/early November in most areas.
However, because localised rain is so important for phoma leaf spot development of phoma, crops should be walked regularly to monitor for signs of infection.
Sprays are usually applied when 10-20% of plants are affected and again when re-infection is observed. Crops should be monitored for re-infection to determine if, or when, a second spray is required.”