1 October 1999

Communicate your desires

By Andrew Swallow

YIELD, uniformity and skin finish all start with seed management. Seed growers and commercial customers need to work together to get it right, says SAC potato specialist Rob Clayton.

"The first thing a commercial grower needs to do is go and talk to his seed producer. Face to face discussion is unbeatable and it pays to get into the crops you have an interest in. Have a look round – make sure the seed growers outfit is up to the standard you would expect."

Equally, commercial growers should be asking for an appropriate quality of seed in the first place. Orders should be placed early enough to avoid compromising handling and dressing, and if the intended ware market has a tight size specification then tighter seed-grading than the standard 35-55mm may be needed.

"Uniformity is determined by the seed spacing and the number of eyes, which is ultimately down to seed size. So the tighter the grade of seed you get, the more manageable the field as a whole is going to be. For example, with a high out-turn pre-packing variety such as Estima you should be thinking about a 10mm grading band, say 35-45mm," he says.

A manageable level of disease is also desirable. "It is unfair to expect completely disease-free seed. Miracles do happen, but we cant all get access to disease-free seed. Setting tolerances is a better bet."

Checking tubers for skin diseases when test digging, looking at the seed inspectors report, and asking what the roguers found will help reveal potential problems.

"Growers can get a lot of background information on quality while the seed crop is still in the ground," he says.

Weather conditions during the seed growing season should be considered to anticipate the risk of blackleg. That could be particularly important with susceptible varieties this autumn. "It was wet early on and blackleg was pretty widespread," he warns.

Once the seed crop is lifted, buyers should ask to see a report on quality at lifting, and be confident of the storage conditions it is going into. Refrigerated storage will help minimise disease development. "It needs to be down to 3.5-4C."

Damage at grading must be minimal, and if fungicide needs to be applied, it is best put on pre-chitting, says Dr Clayton.

"If seed has chitted and you try to treat it, a lot of sprouts will be lost and you will end up in a mess. Fungicides have to go on before we see sprouts," he says.

That means growers aiming for earlier markets, planning to chit crops to advance seed, must think about fungicides earlier.

To determine whether treatments are necessary, seed stocks should be sampled, washed and inspected for skin diseases. "The sample should be a minimum of 200 tubers per stock, taken from different parts of the load," he advises.

No visual symptoms of disease usually means no microscopic level of disease, and treatment is not necessary. However, blackleg or skin spot infections may not be visible, so susceptible varieties such as King Edward, Cara, or Russet Burbank should be tested just in case.

"Skin spot can cause blindness in the eyes, and severe seed infections result in complete non-emergence," he warns. Incomplete skin-set, and late lifting increase the incidence.

A phone call to the testing station is advised to ensure samples for testing are taken correctly.

FUNGICIDE APPLICATION ADVICE

&#8226 Sample 200 tubers per seed stock, from different parts of load.

&#8226 Wash and inspect for damage and diseases: skin spot, black scurf (rhizoctonia), silver scurf, common & powdery scab.

&#8226 Generally no visible fungal disease means no need to treat.

&#8226 Test skin spot or blackleg susceptible varieties just in case.

&#8226 If treatment warranted, treat early, pre-chitting, on clean seed.

&#8226 Imazalil (eg Fungazil) key active for silver scurf control.

&#8226 Monceren (imazalil + pencycuron), Risolex (toclofos-methyl) and Rovral (iprodione) all adequate on stem canker.

&#8226 Liquids preferable to dusts – more even coverage and operator safety.

&#8226 Roller-table application preferable to on planter.