7 March 1997

Reap rewards from fresh look at dressings

Seed potato dressings offer valuable protection against diseases in ware crops. But which should you choose? Lincs-based agronomist David Hudson offers a guide

POOR prices in 1996/97 and worries about future profits are forcing growers to re-examine production costs, says Mr Hudson, crop consultant to the Sutton Bridge producers group whose members grow about 2800ha (6900 acres) of potatoes.

"There is also pressure from our buyers to reduce chemical inputs in crop production," he says. "The key question is what scope is there for reducing rates or even doing without dressings for this seasons plantings?"

The aim of dressing is twofold, he explains. "Our objective is to protect, or hopefully increase ware yield and quality. Quality includes tuber size, shape, uniformity and maturity – all of which can be affected by rhizoctonia.

"We may also hope to reduce the transmission of silver scurf, skin spot, black scurf and powdery scab from the seed to the ware crop."

Not all disease comes from the seed. Spongospora subterranea which causes powdery scab, and rhizoctonia, which shows up as stem canker and black scurf, are common soil-borne fungi.

"Silver scurf is now strongly suspected as being soil-borne," adds Mr Hudson. "So even if we do manage to control the seed-borne element of a disease, there may be enough soil inoculum to spoil our ware crop anyway." The longer the potato history of the field and the closer the rotation, the greater the risk of infection.

The role of good husbandry should not be forgotten, he advises. "Silver scurf and to a great extent rhizoctonia can be controlled to commercially acceptable levels by a combination of sensible rotation, buying clean seed from known sources, and planting in kind soils. The minimum interval between successive crops should be six years – but the longer the better.

"Early harvesting and tight store management are the most important ways of controlling skin blemish diseases, though its worth remembering that early lifting in warm soils may increase dry rot."

Previous seed treatments are also often overlooked, he believes. "Dressings with more appropriate materials may already have been applied. Knowing which have been used could save cost and simplify planting. It is also important to establish what fungicides may have been applied earlier because some combinations of dressings, for example dusts on top of dusts, are not recommended." Imazalil treatments should be no closer than eight weeks, he adds.

There is very little published evidence relating to the value of cut rate seed dressings, says Mr Hudson. "Practical experience with Rovral dressings against rhizoctonia, Fungazil against silver scurf and Mazin against powdery scab indicates that reduced rates with these diseases are not worthwhile. However with Rizolex and Monceren half rates are very effective."n


Table 1: Seed potato dressings approved for use at planting on tubers with eyes open

ProductDiseases controlled

Monceren (pencycuron) dust & liquidSeed-borne rhizoctonia

Rizolex (toclofos-methyl) dust " "

Rovral WP (iprodione) " "

Monceren IM (imazalil + pencycuron)Seed-borne rhizoctonia plus   dust and liquid silver scurf

Mazin (maneb + zinc oxide)Reduction of severity of seed- borne powdery scab

Note: Other products such as Rizolex Nova (prochloraz + toclofos-methyl) and Fungazil (imazalil) may only be applied before chitting.


Table 2: Effect of diseases

FungusDiseaseEffect on crop

Helminthosporium solaniSilver scurfSpoils stored tuber appearance*

Rhizoctonia solaniStem cankerPoor tuber size, shape, uniformity & maturity

" "Black scurfUnsightly tubers*

SpongosporaPowdery scabScabs and mis-shapen subterranea tubers & loss of

saleable yield

Note: There are no dressings for blackleg control.

* In washed samples. Condition less important for unwashed markets.


Table 3: Treatment guidelines (based on washed sample)

Disease symptomsAction

Black scurf not obviousUse nothing

Black scurf obviousUse half rate Monceren or Rizolex

or full rate Rovral

If crop destined for pre-packing and storage & black scurf obviousUse full rate Monceren IM

If powdery scab seen & soil known to be free of the fungusUse full rate Mazin

Note: Combinations of dusts are not recommended.

Well chosen dressings can help ward off diseases like black scurf (left) and silver scurf.