27 September 1996

sprays well worth it

OSRlight leaf spot

LIGHT leaf spot control in Scottish oilseed rape crops showed significant differences, according to fungicide and variety trials last year.

Treating light leaf spot last season nearly doubled yields in Aberdeenshire, says Keith Dawson of Perth-based CSC Cropcare. And although most Scottish growers routinely spray for the problem, more can be done, he reckons.

"The average farmer is catching up. But there is a tendency to leave it too late and then the weather closes in. Our advice is to go as soon as you have three to four leaves. It is better to go early rather than miss the chance."

A bonus of containing light leaf spot is increased oil content. "We have had an extra 1% from control."

The trend to earlier sowing, especially after set-aside with vigorous varieties like Synergy and Commanche, means many crops are well advanced going into winter. The resulting micro-climate makes them much more prone to infection, he warns.

"We have tested a number of products and found tebuconazole, as in Folicur, has been the best." On three out of four varieties in the Aberdeenshire trial it boosted yields the most (see table).

Apart from controlling the disease, the fungicide appears to modify the crop canopy. "It seems to increase root growth at the expense of leaves.

"We get greener leaves and a more prostrate habit. The more compact plants are less susceptible to winter kill and the micro-climate is less favourable for light leaf spot.

Good controllers

"Punch C, Alto and Plover are all pretty good at controlling light leaf spot and Plover is particularly good on phoma. But they have no effect on canopy structure, and that is reflected at harvest where the yield increase is not so big."

Best strategy, he advises, is to apply an autumn spray at one-third to one-half normal dose rate at the first opportunity. "Very responsive varieties like Synergy, Commanche and Falcon need a higher rate than, say, Express, which is more resistant to light leaf spot."

Other factors influence disease risk and hence dose rate, he adds. Infected trash from previous oilseed crops nearby can be a potent source of inoculum. "It can be like a ring of death."

Depending on results growers should repeat the treatment after Christmas or in early spring using a further half to one-third dose according to the risk.

With light leaf spot widely resistant to MBC fungicides, Dr Dawson says growers can expect better results from straight flusilazole (as in Sanction) than Punch C (carbendazim + flusilazole). "It is best to leave the MBC out."

The trials also point to the value of including an adjuvant. "We found both Arma and LI-700 helped, but Arma gave significantly better results.

Oilseed rape yields in CSC Cropcare 1995/96 fungicide trial in Aberdeenshire t/ha (cwt/acre)

UntreatedFolicur+Arma inSanction+Arma inPlover+Arma

autumn &autumn &Folicurin autumn &

in springin springin spring

Falcon2.91 (23.2)5.27 (42)4.87 (38.8)4.27 (34)

Synergy2.65 (21.1)5.11 (40.7)4.63 (36.9)4.00 (31.9)

Express3.50 (27.9)4.97 (39.6)4.65 (37)4.45 (35.5)

Capitol3.26 (26)4.48 (35.7)5.32 (42.4)4.86 (38.7)


Oilseed rape yields in CSC Cropcare 1995/96 fungicide trial in Aberdeenshire t/ha (cwt/acre)

UntreatedFolicur+Arma inSanction+Arma inPlover+Arma

autumn & inautumn & Folicurin autumn & springin springin spring

Falcon2.91 (23.2)5.27 (42)4.87 (38.8)4.27 (34)

Synergy2.65 (21.1)5.11 (40.7)4.63 (36.9)4.00 (31.9)

Express3.50 (27.9)4.97 (39.6)4.65 (37)4.45 (35.5)

Capitol3.26 (26)4.48 (35.7)5.32 (42.4)4.86 (38.7)