61

PERENNIAL WEED CONTROL IN CALIFORNIA ORCHARIB

C. L. Elmore, D. E. , A.H. Lange, L. L. Buschmann and R. B. Jeter University of California

There are approximately 1,000,000 acres planted to orchard crops in California. Of this acreage a large percentage is infested with one type or other of perennial weeds. The four predominant perennial weeds are Johnson grass, field bindweed, Bermuda grass and nutsedge (nutgrass). All of these pests post specialized problems in different areas and different crops. Recently the spotlight has been brought to bear on this problem because of the need for mechanization of harvesting mechanically and because of the interest in non-cultivation of orchards. In the first situation it can readily be seen that perennial weeds can interfere with land preparation for such crops as walnuts and almonds where the nuts are dropped onto a smooth, rolled surface in preparation for sweeping and pickup machines. Also in the same situation when large clumps of weeds such as Johnson grass clunps are present around trees there are large quanti­ ties of nuts or fruit as in case of prunes, lost in these weed infestations. In addition it is difficult to near impossible to use sweepers and pickup machines effectively when perennial weeds are present. Perennial weeds also pose a problem in the operation of pickup machinery. Other advantages of control of perennials can be listed such as compe­ tition for moisture, nutrients, etc., and harboring of insects, rodents, and others. In the second situation as in strip treating an orchard or full coverage treatment for annual weeds it is possible to trade a minor problem for a major one. In freeing the perennial weeds of competition from annuals, weeds such as bindweed, Johnson grass and Bermuda spread rapidly and growers may become somewhat disillusioned with chemicals for weed control in orchards. If a grower has perennial weeds in his orchard he should either control these weeds first or in conjunction with his annual program. J:_ohnsongrass : Since a good part of our program has been directed at Johnson grass the principle part of this presentation will be the control of this very important orchard pest. Other than cultivation, a principle means of controlling Johnson grass, there is dalapon and the organic arsenical group, disodium methylarsonate and monosodium methylarsonates. The latter materials have received a large amount of grower interest in the control of Johnson grass in crop and non-crop areas. Experiments were conducted this year in orchards with dalapon and MSMA. The material MSMA was applied, as treatments as follows: (A) at 4 week intervals at the rate of 4 pounds actual per acre for four applications; two applications at 8 pound actual per acre and; one application at 16 pounds per acre. 62

These trials were designed to determine if repeated applications would be more effective than a single application at equivalent total rates. In other treatments in the same experiments dalapon was applied at four pound actual per acre for four applications at 4 week intervals. Also dalapon was applied at the recommended 10 lb/acre rate. In other greenhouse experiments with fruit tree seedlings the material MSMA and dalapon were applied as a soil drench at 16, 32, and 64 ppm. On Rio Oso Gem peach, Mahaleb cherry, Royal apricot, Myrobalam plum, French prune and sugar prune, no injury was found from MSMA. Cherry, peach, plum, peach, apricot, and French plum were injured with dalapon at 64 ppm. In some species as low as 16 ppm dalapon in the soil solution caused injury.

Recent non-crop area studies showed OOMA and MSMA gave essentially the same control on a pound per pound basis, with MSMA giving slightly more con­ sistent results.

In these studies I'1SMA at the repeated four pound rate consistently gave better topkill control in all experiments. Two applications of 8 pounds was quite effective in some experiments however in most the regrowth was more apparent with higher rates. A single 16 lb/A rate was virtually ineffective. Repeated low application rates of dalapon were consistently better than a single 10 lb/A rate. Although somewhat speculative, these results could be explained on the basis of the rapid destruction of conducting tissue due to the fast burn with high rates. Bermudagrass: Dalapon or repeated oiling have generally given tne best control of Bermuda. grass. Repeated applications of may also prove to be effec­ tive. If paraquat is used, however, the plant products must be handled to comply with current State and Federal regulations covering registration. Bindweed: Bindweed control has remained centered around time of application of 2,4-D, with fall application showing more promise than spring. and have shown promise in non-crop land but because of movement through the soil these have very little potential for bindweed control in orchards.