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Phoma - timing and plant size is everything

At this time of year young rape crops can be at risk of phoma damage and small plants with low resistance scores are most at risk. Catching the disease early is the most important aspect of control and what a difference a year makes!

Cast your mind back 12 months and OSR crops could largely be split into two categories, particularly in southern and eastern regions. Early August drillings found a bit of moisture and got away well in warm soils, but anything planted later effectively became a mid-September drilled crop, growing only when some much-needed rainfall came along.

By contrast, the catchy weather that hampered this year's harvest has meant no shortage of seedbed moisture and allowed crops to get off to a flying start whether drilled in late July or into September.

Weed control decisions have been easier this year too. Residual herbicide activity is more predictable with moist soils, and contact graminicides have been required early in the crop's life to deal with the big flushes of cereal volunteers that are a feature of the season.

The next consideration is disease control, specifically phoma. Anyone who collects weather data at home or on farm will have noted some significant and intense rain events during the 'summer' we've just experienced, but take note of how many days you saw some rainfall locally. With rain falling in 'pavement wetting' amounts around every other day across much of the country, we didn't get far into September before the 20 day threshold that is typically required for phoma spores to mature had been reached. Phoma spores require a few days of dry weather, then another rainfall event to trigger their release, after which new sown crops are at risk of infection.

This is where we find ourselves right now.

With visible lesions recently appearing on crops as far north as Yorkshire, down in Hampshire and across into Herefordshire, think about the varieties you have on the farm. There are some with very high potential for phoma because they don't have high natural resistance. These should be monitored as a matter of priority (see AHDB Recommended List or, if growing varieties not listed, the breeder or your seed supplier will be able to provide this information). Fungicide treatment should be considered if 10% of plants are infected and showing leaf lesions.

We're not short of treatment options, but timeliness is key to protecting crops. The leaf infection and loss of green area is not the issue, but if phoma is allowed to move into the stem it will restrict the flow of resources (water and nutrients) up into the canopy. Infected crops will inevitably underperform and are at an increased risk of lodging later on.

Similarly, with suitable conditions for establishing OSR stretching into the middle of September there are plenty of small plants out there at an increased risk of uncontrolled phoma developing into damaging stem cankers. The speed with which the disease transfers from the leaf down into the stem is determined by the distance it has to travel. Hence bigger plants with longer petioles afford greater flexibility around treatment timing. 

Monitor your fields carefully over the coming weeks, prioritising small, slow growing crops and anything with lower natural defences for treatment as soon as possible if thresholds are reached.

​Paul Cartwright
Crop production specialist

For specific advice for your business related to this blog or any other aspect of crop production get in touch with Frontier.

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Tuesday, 22 June 2021

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