24 April 1998

Volunteers pots best nipped in the bud

WARMER winters and increased riddle size mean a bigger volunteer potato headache for sugar beet growers. Unless the problem is well controlled at every opportunity it can escalate, claims Dow AgroSciences Donald Westwater.

A company trial at Morley Research Centre last year highlighted compelling reasons for preventing a build-up. Just five volunteers/sq m in sugar beet, by no means uncommon, depressed yield by nearly 16t/ha, worth about £500. But herbicide control costs only £70, he maintains.

Substantial problem

The size of the groundkeeper problem in beet is substantial, says Mr Westwater. At least 19% of the 199,000ha (492,000 acres) is in the same rotation as potatoes.

Typically this is potatoes, wheat, sugar beet, wheat. After potatoes an average of 12.5 tubers/sq m are left. This is more than in the past due to increased riddle sizes which means more small potatoes are discharged by harvesters, he explains.

"Winter wheat competes quite strongly with volunteer potatoes so 9/sq m are left assuming the crop receives no Starane spray to control them," he notes. "But the following sugar beet is nowhere near as competitive, so there can be a five-fold increase in numbers by the time the beet is harvested. This is only reduced to 31/sq m in the next wheat which means that without control measures volunteers build year on year."

Apart from significantly reducing sugar beet yield, uncontrolled volunteers help perpetuate PCN and blight and impair varietal purity, says Mr Westwater. Autumn cultivations to bring escaped tubers to the surface can cut numbers by 75% through frost kill in hard winters. But this winter was mild, he notes.

Just how effectively the problem can be tackled was shown in the MRC trial where tubers were planted in sugar beet at five, 10 and 20/sq m. The crop had a normal herbicide programme and two Dow Shield (clopyralid) treatments of 0.5 litres/ha with standard rate Nortron (ethofumesate).

The first Shield spray was on May 16 when most of the potatoes had emerged and were 5-7cm tall. The second was on May 29 when the treated haulm, mostly regrowth, was 5-10cm and the untreated crop about 20cm.

The dual treatment decreased volunteer numbers by over 75% and markedly reduced survivor vigour so that even the highest tuber plantings depressed beet yield only marginally.

Dow agronomist, Rene Pollock, says the result showed that for each volunteer eliminated the yield response was about 3t/ha, worth about £95. This means that whenever volunteer populations exceed an average of 1/sq m treatment is economically viable. &#42

Uncontrolled volunteer potatoes can cut yield in sugar beet, harbour blight and perpetuate potato cyst nematode. Well chosen herbicides can help contain them.