Early defences

6 October 2001




Early defences

Dont be caught out by an early phoma epidemic. Gilly Johnson asks how growers can protect their rape

JUST two years ago, you would not have been reading an article on rape disease control, here on the pages of Crops, in early October. November perhaps – but not this early. Thats because the key disease, then light leaf spot, turned up later in the season.

Today, disease threatens your crop earlier. And its become known that the main enemy has changed: its phoma, not light leaf spot, for growers in England. But what isnt widely appreciated is that the risk period has been shunted forward even earlier – into October. And because phoma has the ability to arrive sooner, its a greater threat to yields, taking up to 0.75t/ha off your tonnage.

Whats going on? The explanation for phomas rise in importance could be something to do with the fact that it is not just one pathogen. Its at least two, and possibly more, suggests Dr Bruce Fitt, of IACR Rothamsted.

Two strains have been identified so far: named the A and B types. Plant pathologists can distinguish between them by inspecting the tiny fungal germ tubes; one strain has straight tubes, the other has curly ones. In the field, growers could make an educated guess as to which strain is causing the problem, although its not easy. The A type is dominant in England, and this one is associated with early leaf spots, which unfortunately are linked to the greatest damage.

The B strain also shows leaf spotting, but these spots may be cracked at the centre, and display fewer little black dots within the spot (see picture). More common in the south than the north, the B strain tends to appear in the spring and so is potentially less damaging. "Most infected rape crops would show both types of phoma," says Dr Fitt.

As yet, the scientists dont know whether varietal resistance is different for the two strains. But this hypothesis could help explain why phoma resistance is proving hard to maintain in varieties.

So what should you be doing to protect rape profits? First, identify what level of varietal resistance you have (table 1). This does not give foolproof protection, but knowing the varietal risk is a good start. Even moderately resistant rapes, rated 7, could benefit from a phoma treatment if the disease arrives early.

Its tricky to predict disease risk, before the spots can be seen. Phoma spreads after ascospores germinate and infect young leaves, but theres a gap of 2-3 weeks between this happening, and visible signs of infection. Ascospore germination is triggered by warm temperatures and rainfall, and if theres a nearby source of ascospores such as crop debris in a neighbouring field. Scientists are working on a prediction service (see box) but for the time being, checking crops regularly and spraying when disease is sighted remains the best defence.

Early epidemics – which means spots appearing between now and late October – are the ones which justify spraying, says Dr Pete Gladders of ADAS Boxworth. When phoma appears later on, late in October and in November, then yield loss can shrink to 0.1t/ha, according to trials at the Boxworth site, which is in the heart of phoma country. Spraying against phoma alone is hard to justify at this level of return – roughly equivalent to the cost of a half-rate fungicide application, he says.

Of course, if theres light leaf spot in the disease mix as well, this swings the balance in favour of treatment; potential yield response then rises to 1-1.5t/ha. But later sprays are effective here, and light leaf spot also puts spring fungicide sprays back into the frame.

Once youve taken the decision to spray on phoma, then aim to go in at least twice, says Dr Gladders. "One treatment alone can be worse than doing nothing; top up with another spray 4-6 weeks later. It seems its not just a case of phoma control; the yield response comes from better plant growth from early autumn programmes."

"If you have to prioritise treatments between different crops, then small plants are most at risk and should be sprayed first."

&#8226 A phoma tracker service, giving warnings of infections, can be found on www.syngenta-crop.co.uk

Autumn disease control

The threat

&#8226 In England, phoma and possibly light leaf spot

&#8226 In Scotland, light leaf spot

Together these diseases can cut yields by 1.5t/ha

Symptoms

Phoma shows leaf spots with black picnidia visible. Early epidemics – where spots are seen before the end of October – are most damaging and lead to higher numbers of affected stems later in the season

Light leaf spot shows bleached spots but without the black dots. Incubation of leaves in a plastic bag, in a warm place, will give prior warning

Timing

Spray as soon as possible when 10-20% plants have phoma spots, with a follow up spray 4-6 weeks later. If phoma appears later at the beginning of November onwards, treatment may not be justified unless light leaf spot is also present, when an autumn/spring programme may be required

Thresholds

For phoma, its not so much how much disease is there, but when it arrives in the crop. Early infections will require spraying.

For light leaf spot, treat if any symptoms can be seen in autumn/winter

Product choice

For autumn phoma plus light leaf spot, Punch C (flusilazole with carbendazim); or for phoma only, Plover (difenoconazole), both used at half-rates in sequences. Plover can be used safely at the 2-leaf stage, says the manufacturer

Password to control

Scientists are pulling together a rape pest and disease prediction service, which will include phoma, under the acronym Password. This builds on a light leaf spot prediction system, which can already be accessed at www3.res.bbsrc.ac.uk/leafspot – go to the growers pages. The model will include a number of different variables including weather, sowing date, variety and regional risk.

Funding for the project is from the HGCA levy, Government and industry partners. Group chairman is Dr David Ellerton of the distributor group ProCam. The plan is that eventually users will pay to access the Password service.


See more