7 February 1997

Age seed for early harvest

Manipulating seed potatoes in store before planting can help avoid quality problems. Edward Long reports

CAREFUL management of potato seed before planting can advance harvest by up to 14 days, so minimising the risk of skin diseases. But conditions in the chitting house need controlling carefully.

"Clearly there is a need to harvest earlier to avoid trouble," says ADAS potato specialist Paul Dover. "The big supermarkets now take a large and growing slice of the total crop, so it is in the growers interest to do everything possible to minimise diseases such as silver and black scurf, and to a lesser extent skin spot."

Many of the popular and widely grown varieties, such as Maris Piper, Fianna and Cara, are naturally late maturing. The resulting late harvest aggravates the risk of skin trouble. That could explain the growing use of second earlies, like Estima, in a maincrop role – allowing earlier lifting for storage. Earlier harvesting also means lifting in easier conditions, with tubers suffering less mechanical damage. Potatoes coming out of warm soil are also less likely to suffer bruising, Mr Dover notes.

Adequate skin set will also need improving in late maturing varieties. ADAS says the factor which most influences this is the stage of maturity at the time of desiccation. Trials at ADAS High Mowthorpe in Yorkshire have shown this is far more important than the type of chemical used.

Advancing maturity requires careful manipulation of seed in store. A total of 250-300 day degrees to advance the harvest by two weeks should be the target, possibly pulling the crop out of the danger zone in a wet season, he says.

"Ideally around 250 day degrees are needed from the time of the dormancy break. If 350 is exceeded seed can become unmanageable due to the risk of damaging brittle sprouts. Crops established from over-aged seed are also more prone to stress, particularly in varieties which are already susceptible to stress, such as Nadine and Record," Mr Dover warns.

High light levels

"For anything over 250 day degrees, light levels need to be very high if sprouts are to be sufficiently robust to withstand planting knocks. If some are lost the whole object of ageing is lost and growers could be worse off with increased variability in the crop."

Potatoes are sensitive to bad soil conditions and must be planted without creating compaction. This often dictates the start of planting. If a traditional calendar date is used, subject to seed-bed conditions at the time, chitted seed is invaluable. Last year crops planted with chitted seed outperformed unchitted ones. Temper-atures in April and May were relatively low and seed-beds dry, yet chitted seed got away earlier.

Mr Dovers target is to achieve a crop with 30-50% natural die-back at the time the haulm has to be killed off. If the crop is usually green when the sprayer starts work there is advantage to be gained by increasing the day degrees.

More robust sprouts

"But the way this is done is also important. Seed which is aged slowly at a moderate 8C is likely to produce more robust sprouts than seed subjected to a higher temperature for a shorter time. So if the target is for 240 day degrees at 8C, with 4C a day over the 4C base, the job will take 60 days from the time of dormancy break. But experimental evidence suggests tuber-to-tuber variation is minimised by breaking the dormancy at 10-12C.

"So the ideal scenario would be to warm seed up prior to the required date of dormancy break, then hold the temperature at 12C until all the tubers show signs of waking up, then cool the store down and maintain a steady 8C until the intended planting date."

Growers without refrigerated facilities in the seed store should aim to get tubers to the target age by mid to two-thirds of the way through the anticipated planting period. So if the aim is to plant between Apr 5 and 15, the day degrees target should be reached by the 10th of the month. &#42

Ageing potato seed before planting speeds harvest, cuts skin blemishing diseases and minimises damage. But temperatures need handling with care, warns ADAS consultant Paul Dover.


CHITTING TIPS


&#8226 Age seed to speed harvest and avoid disease and damage.

&#8226 250-300 day degree target.

&#8226 Over 250 day degrees needs light to strengthen chits.

&#8226 Leads to bolder sample.

&#8226 Age slowly for best results.

&#8226 Hold in refrigerated storeonce aged if possible.


Seed size little influence on harvest date


Seed size has little influence on the harvest date for potatoes, unless it leads to problems with planting depth. For best results ensure an equal depth of cover for all sizes of seed, taking particular note to avoid excessively deep planting of the smallest fraction, ADAS potato specialist Paul Dover advises. Close grading to within a maximum 10mm (0.4in) range will minimise the risk of uneven soil cover and maximise the chances of achieving a uniform crop, he says.