Growers do not have to till diverted vineyards as much. This is a cost savings that can improve grapevine growth and yield capacity.

A significant acreage of Thompson Seedless vines in the San Joaquin Valley are expected to be enrolled in the federal raisin diversion program this season, which means raisins cannot be produced from them. In return, enrolled producers will receive surplus raisins as payment for not producing a crop this season.

This is designed to reduce the surplus of raisins.

University of California Cooperative Extension farm advisors Bill Peacock (Tulare County), George Leavitt (Madera County) and Stephen Vasquez (Fresno County) have put together a management plan for growers who spur prune their Thompsons to preclude them from producing grapes.

The farm advisors say while spur pruning will remove most of the crop, it may be necessary to remove flowers by hand in April.

The diversion program is an opportunity to strengthen vineyards with good irrigation and nutrition. However, don't let vines become too vigorous, advisors say, and don't over irrigate. While the diversion program is an opportunity to reduce costs, don't let mildew and pests become a program for neighboring vineyards producing a crop.

Recommendations

The farm advisors recommend leaving six spurs or less on weak vines; six to eight on average vines and eight to 10 on vigorous vines.

While the diversion program is an opportunity to strengthen weak vines and reshape older vines, don't make large cuts to invite bot canker or eutypa dieback. Arm positions should be maintained and vines should not be cut back to the head or crown.

Spurs should be short — a base bud plus one bud (one-bud spur). A base bud is no more than a quarter inch away from the old wood. Longer spurs may be necessary on very vigorous vineyards to divert vigor into more shoots, but this will increase flower clusters.

The diversion program does not permit more than four to five clusters per vine. Flowers should be removed by mid-April when shoots are 12 to 14 inches long. Delaying this until full canopy increases cost.

Growers do not have to till diverted vineyards as much. This is a cost savings that can improve grapevine growth and yield capacity. However, weeds should not be neglected, especially noxious weeds like johnsongrass, bermudagrass and morningglory.

Nitrogen fertilization can be eliminated in most cases, especially in vines on deep sandy loams or fine sandy loam soils. However, weak vines on sandy soil may still require some nitrogen to sustain or improve vine growth or production capacity.

Adequately irrigate weak vines and those on sandy soils. Don't stress a weak vineyard. Irrigate in August and September. If drip irrigated, apply full schedule amounts to weak vines.

Excessively vigorous vineyards in the diversion program should be deficit irrigated to avoid excessive growth, which can reduce bud fruitfulness and the crop next season.

Delay the first irrigation until early May on shallow or sandy soils and early to mid-June on deep fine sandy loam soils. Irrigate after that based on shoot growth. Extend irrigation intervals. Consider irrigating every other middle, alternating furrows or middles with each irrigation. This prevents loading up entire root zones and stimulating excessive root growth.

Avoid, heavy deep irrigations. If drip irrigated, apply water at half to two-thirds scheduled amounts in vigorous vineyards.

While mildew and pest control can be minimized, if rain occurs during budbreak until mid-April control measures will be necessary for phomopsis cane and leaf spot, especially in vineyards with a history of phomopsis.