Oilseed rape growers could save money this autumn because of a delayed threat from disease following the very dry summer, which may prompt some to cut out a fungicide spray.

The threat from phoma disease is developing about one month later than last autumn, meaning that those growers who spray twice in the autumn could just treat once, or possibly not at all.

Faye Ritchie, a plant pathologist at crop consultant Adas, says phoma is likely to reach spray threshold levels only in late October or early November after the spell of very dry weather.

This means that one fungicide spray could be aimed at phoma and at the same time target the potentially more serious light leaf spot in November, when control for this latter disease is often applied.

“Growers should be able to get away with one spray this autumn rather than two, especially with varieties which have good phoma resistance,” she told Farmers Weekly.

See also: Light leaf spot: All you need to know about this OSR disease

Phoma disease forecasters measure the number of rainy days from the start of August. When the total  reaches 20 days, phoma disease spores are released and can then infect plants some two to three weeks later.

Light leaf spot with its characteristic grains-of-sand appearance © Blackthorn Arable

Rain day target

The 20-day target was hit on 10 October at Boxworth in Cambridgeshire this autumn compared with 11 September last year, and this means many crops are a long way off the spray threshold of 10-20% of plant infected.

“We usually see phoma infection before mid-October, but we are only likely to see spray thresholds hit in the next week or two,” Dr Ritchie says.

Phoma disease, which is encouraged by warm and wet weather, can cut yields on average by 0.3t/ha and in severe cases up to 1t/ha, so that could be approaching 30% of an average-yielding rapeseed crop.

Growers are advised to spray for phoma when 10-20% of plants are infected and for light leaf spot as soon as the disease is seen. The latter disease is difficult to diagnose in the field and is much easier to spot when a few leaves are incubated in a polythene bag at room temperature for three to four days.

Lower risk

The risk from the potentially more damaging light leaf spot is also likely to be lower following reduced disease levels being seen in the spring in both 2017 and 2018 which resulted in less inoculum in oilseed rape trash after harvest. The disease is encouraged by wet and cool weather.

“A late phoma spray could also be a light leaf spot spray if the grower chooses a product with good phoma and light leaf spot activity,” Dr Ritchie says.

Phoma appears as bleached patches on the leaves covered with dark spots, and can move to the stem and so cause cankers which can stunt growth or kill plants, while light leaf spot produces pale green mealy lesions and tiny white spore droplets which tend to look like grains of sand.

Fungicide control of phoma is largely based on two azoles, prothioconazole and difenoconazole, while prothioconazole and another azole, tebuconazole, are key fungicides to control light leaf spot. SDHI products can also be used (see below).

One-spray strategy 

Ryan Hudson, regional agronomist with Velcourt, says many growers sprayed twice last autumn with phoma appearing so early, but this year one spray will probably be sufficient to cover both phoma and light leaf spot, or if the weather stays dry then no autumn fungicide spray may be needed.

“The disease pressure is very low, and we are only seeing the odd phoma lesion compared with last year when we saw phoma in September,” he says.

Mr Hudson’s strategy is to use tebuconazole for strong healthy crops as this azole has some plant growth regulatory effects, while he will use the more effective and expensive prothioconazole for smaller crops of more susceptible varieties.

“For varieties in the east with good disease resistance and with big plants then there is the potential to delay fungicide application until the new year,” he adds.

Fungicides with activity against phoma

• Prothioconazole (Proline)
• Difenoconazole (Plover)
• Boscalid (Filan)
• SDHI penthiopyrad + strobilurin picoxystrobin (Refinzar). This product is being withdrawn from the market and any product needs to be used up by 30 November.

Fungicides with activity against light leaf spot

• Prothioconazole
• Tebuconazole (eg Folicur)
• SHDI boscalid + strobilurin dimoxystrobin (Pictor)
• Refinzar. Must be used up by 30 November.

Disease threat changing on the South Downs

David Taylor (left) and Mike Thornton

Sussex grower David Taylor has only once sprayed for light leaf spot in the autumn in recent years although the disease could become more important than phoma in the future.

Farming on the South Downs above Brighton, he grows 70ha of oilseed rape and in late October his well-grown crops were showing only occasional phoma lesions after very dry weather in July, August and September.

“Last season we did not spray for phoma or light leaf spot but then we had just had a break from oilseed rape the year before,” he told Farmers Weekly.

Mike Thornton, head of crop production at agronomy group ProCam, added that with better varietal resistance against phoma and as the farm is only four miles from the sea, the wet-loving light leaf spot could become more of a key disease.

“The phoma risk is considerable lower this season and we are not seeing the disease in this area, while light leaf spot is possible later in the autumn,” he said.

His strategy will be to use prothioconazole or a prothioconazole/tebuconazole mix to give good control and protection over both diseases this autumn.

Mr Taylor is growing the Clearfield variety PT279CL for the second season, mainly as he can cut out a pre-emergence herbicide and thus save costs, and last season the crop matched the farm’s average yield of 3.3t/ha.

This season the crop on the 500ha Housedean farm between Brighton and Lewes was drilled on 20 August, sprayed once with a pyrethroid insecticide for cabbage stem flea beetle, and had grown strongly by late October.