Careful crop walking required for effective Phoma control

Phoma risk this autumn is high and oilseed rape crops will need careful crop walking this autumn in order to assess each individual field’s needs, to time fungicide treatments correctly and to ensure effective protection against Phoma, which can cause up to 0.7 t/ha yield loss, according to ADAS Plant Pathologist, Dr. Peter Gladders.

"Despite the wet conditions in August and September which indicated the potential for very high disease pressure from Phoma, we are seeing variable crop stages and variable disease levels across the country. Some rape crops drilled in early August are already very large, whilst others drilled in early September have 3-4 leaves. Others drilled even later are struggling through at cotyledon stage. The crop stage is important when it comes to deciding when and what to spray. Small crops are very vulnerable to disease and can be killed outright by Phoma, but they are difficult to treat as they present too small a spray target. Most crops should receive their T1 fungicide when they are at the 3-4 leaf stage. Larger crops will need a treatment that gives both Phoma and plant growth regulation at this T1 stage," says Peter.

Dr. Gladders reports that Phoma leaf spotting is being reported in many parts of the country, but incidence to date is variable according to the local rainfall pattern. "This variability means that advisors need to be out there this year inspecting crops regularly in time for the T1 spray. They should be assessing not just the size of the plants, but also disease symptoms and crop location. Crops close to rape stubbles are at higher disease risk for example. Some crops have already reached threshold and have been sprayed with their first spray."

Peter is concerned that some growers may not be giving disease control in rape the priority it deserves this year, as they are focusing their attention on drilling wheat. He points out that the 0.7 t/ha yield loss that can be attributed to Phoma on moderately susceptible varieties is still worth over ￡200/ha and that rape disease control strategy needs to move further up the priority list this autumn.

Oilseed rape crops will need protecting with a two or even three spray fungicide programme and should be treated according to disease threshold, which is when 10-20% of plants show Phoma leaf spot lesions. Dr. Gladders points out that this threshold is more critical if plants are small or when growing susceptible varieties. "Trials have shown a positive response from fungicide treatments on smaller plants right up until Christmas. We have been used to managing disease on moderate to large crops when spray timing and dose are less critical. Smaller crops with high cotyledon infections need a different approach which includes the use of non PGR treatments with good curative activity. With the potential high risk conditions this year, it is more important to keep inspecting your rape crops and to be ready to start the disease control programme at the right time for that crop."

"It is important to continue disease monitoring in the autumn as re-infection will take place and the second T2 spray will need to go on around 4-6 weeks after the T1 treatment. If the weather is mild in the next month or two, this interval will be tighter at just 3 weeks. If temperatures post -T1 treatment are cool, the interval will be longer. By the T2 timing many more crops will benefit from a robust fungicide which also has PGR effects such as Caramba."

"Caramba offers good disease protection against Phoma and some activity on Light Leaf Spot and will be a good T1 treatment on those healthy, robust and larger crops of rape. An application will improve the rooting structure of the oilseed rape and provide the first step in canopy manipulation for larger crops. Very small crops under stress will not benefit from this PGR effect early on and so Caramba will be a more suitable option when the crop is more robust," says Paul Rawson, BASF Oilseed Rape Product Manager.

"Crops should be monitored carefully for leaf spotting and be treated within two weeks of the symptoms appearing. The fungus should be prevented from spreading down the leaf to the leaf axil and then to the stem, where the damaging canker phase occurs. When the threshold is reached, growers should be prepared to spray Caramba. Commercial usage and trials show that Caramba matches all other fungicides in terms of Phoma control and yield," advises Paul.