7 August 1998

Liquid not dust and only if disease means you must

Fungicide and phosphate potato seed treatments and a

better understanding of rhizoctonia were key issues at an

agronomy open day organised by Profarma in

Shropshire last week. Andrew Swallow reports

ROUTINE use of potato seed treatments contravene crop assurance protocols, and may be an unnecessary expense, says Profarma.

Applications should only be made in response to an assessment of seed-borne disease, says agronomist Andrew Wade. "Every lot of seed that goes in the ground should be assessed for disease. To start with that can be as simple as washing it under the tap. Only then will skin disease such as scurf and black dot show up."

Such an approach has helped the firms agronomists identify more seed that could be planted untreated, adds colleague Serena Eustice. Where treatment is justified, timing and method of application also need care, she adds.

"Liquid treatments are more target specific, and use lower doses of active ingredient to achieve a better result."

Trial results from the firms select agronomy sites at P M and R M Belchers Tibberton Manor farm, in Shropshire, back those claims. Full-rate Monceren IM dust (imazalil + pencycuron) at planting produced 61t/ha (25t/acre) of early baking variety Marfona, compared with 66t/ha (27t/acre) untreated, and 69t/ha (28t/acre) from plots given a full-rate Monceren IM Flowable liquid application at planting.

"And whats more, there is loads of black scurf on tubers from the dust treated plots, because the dust treatment is so hit and miss."

Yields are further improved if liquid applications are split, with half-rate pre-chitting, followed by a half-rate spray on to tubers at planting. Rizolex Flowable (tolclofos-methyl) applied half-rate at planting and half-rate pre-chitting gave 80t/ha (32t/acre), compared with 68t/ha (27.5t/acre) from a full-rate at planting on the Tibberton plots.

A full-rate liquid treatment costs about £60/ha (£24/acre). "That is less than 1t/ha of marketable yield response," concludes Ms Eustice.

Growers should wash seed and inspect for skin disease before prophylactic treatments, says Profarmas Andrew Wade.