17 October 1998

SPOT OF BOTHER

Oilseed rape disease merits your particular attention this autumn, writes Tia Rund.

CLOBBERING blackgrass may be your number one autumn priority, but take some time out from cereals to concentrate on autumn rape fungicides in whats expected to be a high disease risk season, is the plea.

The trouble is that a lot of whats in the ground is poorly served by natural resistance to stem canker, the most serious disease in England and Wales. NIAB calculates that less than a quarter of the total area planted has a rating of 7 or above – mainly Licrown, Contact and Express. Although classified as becoming outclassed, Express has one of the best levels of canker resistance ever seen, says NIAB plant pathologist Jane Thomas.

Trials carried out by NIAB on behalf of DuPont have shown the value of fungicide treatments in reducing the canker disease index – a combination of disease incidence and severity. "This followed through to yield, though not in any spectacular way," adds Dr Thomas. "The untreated yield of Express wasnt far off the treated yield of Synergy (rated 5) and Nickel (4).

"But, if you dont treat varieties such as Nickel and Synergy, you must expect a drop in yield." Untreated crops will often also lose out on oil content, forgoing on average £3.50/t of additional benefit, she states.

Untreated, even the more resistant varieties will have nearly 75% of their stems infected, but a much smaller proportion will develop the most severe cankers which go right through the stem base and cause premature senescence, she explains.

ADAS plant pathologist, Peter Gladders, believes that fungicides are being used in adequate quantity but are often poorly timed.

High levels of phoma leaf spot in the autumn invariably lead to high levels of canker later in the season, he states, but its six months between spotting the spots and witnessing the economic damage at the bottom of the plant.

The fungus moves down through the plant at the rate of about 5mm a day. "Leave treatment any later than four weeks after the symptoms appear and youre wasting your time," he says.

Because of the longer distance the fungus has to travel, plants with bigger leaves, even under milder temperatures, buy up to three weeks flexibility in spray timing. So target the poorer looking crops first, rather than following your natural inclination to nurture the star performers, advises Dr Gladders.

Monitor all crops on a weekly basis: the disease can leap from zero to 50% phoma leaf spot infection in the space of a week. By the time 25% of plants are affected, you should be taking prompt action, he suggests.

"Two half doses – in November and February – should do a robust job, but if the epidemic starts in October, think in terms of three treatments."

Dr Thomas agrees. On the plots at this years NIAB Seeds and Varieties Day, autumn fungicide sprayed in October ran out of steam a little too soon on Synergy and Pronto. A November spray of 0.4 litre/ha Punch C (carbendazim + flusilazole) with a February follow-up gave best results.

This, albeit unreplicated, demonstration again underlined the flexibility in spray timing afforded by varietal resistance.