Beware late phoma and light leaf spot risk
Oilseed rape growers are being urged to watch out for signs of phoma leaf spot and light leaf spot over the next few weeks ...
Oilseed rape growers are being urged to watch out for signs of phoma leaf spot and light leaf spot over the next few weeks, and treat crops where necessary to prevent any infection taking hold.
While very dry weather in August and September significantly delayed the onset of phoma, recent rainfall combined with mild temperatures means cases are likely to increase, says Hutchinsons head of integrated crop management, David Howard.
“If temperatures remain high, it will also increase the speed of transmission from the phoma leaf symptoms into the stem, causing the more damaging stem canker.”
Assessing risk
Tools such as the AHDB’s phoma forecast (https://ahdb.org.uk/phoma-leaf-spot-forecast) provide a useful indicator of disease pressure and estimated treatment threshold date, but predicting the exact risk and treatment requirements in individual fields can be tricky, especially given this season’s wide variations in crop size and maturity, Mr Howard says.
“Smaller crops are generally more at risk than larger crops as they have less distance for the fungus to travel before entering the stem and forming canker. But, given the higher temperatures forecast, even large crops will be at risk as temperature increases speed of movement.”
The treatment threshold for phoma leaf spot is when infection is seen on 10–20% of plants, however in smaller crops, it may be worth targeting a fungicide earlier, at 10% of plants infected, he suggests.
“Varieties with good phoma resistance (RL rating 7 or above) can delay the arrival of phoma symptoms, and in some cases a single fungicide application against both phoma leaf spot and light leaf spot may be sufficient, but this is not a certainty. Always check crops regularly for disease meeting thresholds.”
Diseases coincide
This autumn’s delayed appearance of phoma means it is likely to coincide with the later light leaf spot timing, allowing growers to potentially tackle both diseases at once, Mr Howard continues.
LLS risk has also been favoured by wetter conditions during October, although it generally prefers slightly cooler temperatures than phoma. “The optimum temperature for LLS is around 16C, so conditions over the last few weeks have been ideal for it.”
LLS is very difficult to manage curatively, so any varieties with a weakness to the disease (RL rating below 7) must be prioritised for protection with a suitable fungicide. Even a high resistance score does not offer complete protection, so those varieties may also benefit from early treatment, he advises.
“The choice of early fungicide should be based around varietal risk scores for both phoma and LLS and the growth regulatory requirement of the crop.”
Prothioconazole and tebuconazole based products, or bixafen plus prothioconazole formulations, generally offer the strongest control of phoma and LLS, while difenoconazole is a useful option against phoma where no PGR activity is required, he says.
“Given the level of variation in crop drilling date and size, PGR requirements will be different across farms and fields. Where crops are significantly advanced, the best approach is likely to utilise a dedicated growth regulator, such as mepiquat + metconazole, as this offers higher growth regulatory activity and timing flexibility, because you are not waiting for the crop’s disease control requirements.”
Large biomass crops (i.e. those with a GAI above 0.8 by the end of autumn) will be best managed with a sequenced PGR in autumn and spring, especially as long-term forecasts currently suggest a mild and wet winter, which may not slow down crop growth quickly, he notes.
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