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UC Pest Management Guidelines


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Potato

Integrated Weed Management

(Reviewed 8/07, updated 5/08)

In this Guideline: More about weeds in potato:

An effective weed management program takes into account the type of weeds present, crop rotation, cultivation, available herbicides, and the competitive ability of the potato crop. Competition from early season weeds will reduce yields if they are not controlled within 4 to 6 weeks after potatoes emerge. Weeds that emerge after vines have covered the rows usually will not compete with the potato crop; however, they may reduce yields by interfering with harvest and can produce seed that will cause infestation of subsequent crops. Weeds frequently become more serious if crop growth is delayed by adverse conditions early in the season. Weed problems can be reduced by establishing a vigorous stand of potatoes.

Weeds and practices to grow potatoes vary in the two major potato-growing regions of California: Kern County and the Tule Lake region. In the Kern County region, soils are sandy or sandy loam with organic matter levels below 1%. Plantings are made from late November until early March for the spring and summer crop and in July for the much smaller acreage winter crop. In the Tule Lake region, soils are largely silty clay loams with organic matter levels of 3 to 15%; plantings are made in May and are harvested in fall. Consequently, not only do cultural practices vary in the two regions, but weed species and herbicide usage as well.

MONITORING

To plan a weed management program, you must know what kinds of weeds are present, which ones are most abundant, and whether their abundance is changing. Regularly survey each field to determine which species of weeds are present and the effectiveness of previous weed control measures. During the early part of the potato season, make weekly surveys. Pay special attention to perennials and, where it occurs, dodder infestations. Also be sure to record annual weeds that go to seed. Where potatoes are rotated, note weeds in the previous rotational crops as well.

To survey the weeds, randomly walk through the field and rate the degree of infestation for each weed species. Sketch a map of the field and mark where perennials occur. Maps are very useful for rechecking the same areas each season to determine if special controls are needed during fallow or rotations. This information will become increasingly valuable over the years in predicting weed problems. Record weed survey information (example form—111 KB, PDF).

ORGANIC WEED MANAGEMENT

Potatoes are a very competitive crop and can be grown without herbicides if soils are moist enough to encourage weeds to emerge before the potatoes. Most weeds can then be removed by a cultivation that is timed just before potato emergence. A few weeds, however, such as nutsedges and nightshades, cannot be controlled by cultivation because they emerge with and after the potatoes and might be best controlled with crop rotation.

The competitive ability of the potato plant varies considerably among the different varieties. If you want to grow potatoes without herbicides, choose one of the more competitive varieties. White Rose, Ute russets, Chieftain, and Red LaSoda are very aggressive and cover rows quickly. The new Russet Norkotah tends to be a bit more upright and varies in cover, while the Tejon White is somewhat less aggressive. Sometimes disease, lack of nitrogen, air pollution, or climatic changes can alter how these varieties grow. Varieties that are harvested green for chipping have less problems with weeds, including nutsedges, invading after early season cultivation because the vine canopy remains until harvest.

Crop rotation and cultivation can both be used effectively to manage weeds.

WEED MANAGEMENT BEFORE PLANTING

Crop rotation is useful in controlling difficult weed problems because it allows for a greater variety of weed control methods. Cultivation during field preparation can greatly reduce weed populations.

Crop rotation. Crop rotations can also help to control disease and nematodes. Cereals, including small grains, corn, and milo, are valuable in rotation; herbicides that cannot be used in potatoes can be used for controlling problem weeds in these crops and after their harvest. Perennial weeds can also be controlled after a grain harvest by discing and watering to encourage weed growth in fall and then applying a postemergent, translocated herbicide to the growing weeds. Dry or irrigated fallowing after May/June harvests in Kern County permits control of many weeds.

Alfalfa is a good rotation crop on marginal potato soils that will not support potato production in short rotations. It shades out many annual weeds and repeated mowing of alfalfa helps reduce infestations of some troublesome weeds. Alfalfa is susceptible to some species of root knot nematode, however, which may diminish its usefulness as a rotation crop. Other closely related crops such as tomato, pepper, and eggplant are not good rotation crops because the available herbicides are similar to those used in potato.

Cultivation. Potato fields should always be tilled after previous crops are harvested to reduce weed seed or propagule production. Further tillage should be done if weeds grow and approach their reproduction stages before potatoes are planted. Usually tillage is less expensive than herbicides.

Herbicides. When carefully used, available herbicides control most annuals that presently infest potato fields, as well as some perennial weeds such as nutsedge, johnsongrass and bermudagrass. Preemergent, soil-applied herbicides usually are mixed mechanically or irrigated into the soil before weeds emerge and kill germinating weed seeds or emerging weed seedlings. Postemergent, foliar-applied herbicides are sprayed onto the foliage of weeds after they have emerged, usually while they are small and actively growing. Choose herbicides that will control the important weed species in your field, and plan to kill weeds before they grow beyond the seedling stage.

The choice of an herbicide depends on weed species present, soil type, cultural practices, cultivar grown, and available herbicides. In many fields, two or three herbicide applications are made during the potato season. A preplant, preemergent herbicide is mixed into the soil during the field preparation and fertilizer application process. Depending on the weeds present, preemergent herbicides also may be applied in some areas 3 to 4 weeks after the preplant application. After planting, usually a combination of two preemergent herbicides is mixed into the soil during or just after hilling. If additional control is necessary after potatoes emerge, certain herbicides may be applied through sprinklers (chemigation). In some areas, vine-killing agents are applied in preparation for harvest. These materials are usually contact, postemergent (foliar) herbicides. Hilling of field-stored potatoes helps control weeds after vines are dead, and postemergent herbicides may be used if additional weed control is necessary.

Many preemergent, soil-applied herbicide treatments are ineffective on Tule Lake's high clay, high organic matter content soils. Herbicide molecules tend to bind with the soil clay and organic matter particles and thus become unavailable for weed-killing purposes. When soil-applied, preplant or layby applications of herbicides are used on high organic matter soils, elevated rates may be required for acceptable weed control. Carefully follow all herbicide label recommendations for use rates on various types of soil.

For nutsedge control in Kern County, EPTC (Eptam) should be applied by March 1. It may be applied preplant or postplant, but two or three applications a season are usually necessary.

WEED MANAGEMENT AFTER PLANTING

Depending on the type of irrigation system used, potato fields can be cultivated at several periods before the row closes over to manage most weeds.

In Kern County, timing the plantings to take advantage of the considerable competitive nature of potatoes is a most beneficial method. On spring potatoes planted in winter, rows can be cultivated during the winter, just before the potatoes emerge, and again up until the 4- to 8-inch stage of growth, if necessary. This removes winter weeds and often the first flush of spring weeds. On February plantings, a cultivation during the period just before emergence to the 4- to 8-inch stage, is usually adequate for season long control of most annual weeds because the potato plants grow so rapidly and outcompete most weeds, with the exception of nutsedges. In summer plantings, the intense heat favors rapid weed growth. Because continuous irrigation is needed to offset the effects of hot weather, cultivation is not a practical method of weed control for these plantings.

In areas where potatoes are irrigated with solid set sprinklers, most growers are reluctant to cultivate after potatoes have emerged. Because the plants are very shallow rooted, an untimely hot spell can severely reduce yields or quality if solid set sprinklers are broken apart to permit cultivation. Potatoes should be kept on the wet side of moist; they transpire a lot of water in hot weather from a rather limited root system. If fields are cultivated, cultivations should be very shallow and done about 3 to 5 days following irrigation. Another concern with tillage is the problem of soil compaction.

In the Tule Lake region, well-timed cultivations can greatly reduce the need for chemical control. Postplant cultivation at hilling eliminates early germinated weed seedlings. This, in effect, reduces the size of weed seedlings that need to be controlled by later cultivation or by postemergence herbicides. A second cultivation before the row closes over may even eliminate the need for chemical weed control. If postemergent chemical weed control is still required, early cultivation will reduce early weed pressures as well as the size of the weeds needing to be controlled. To maximize weed control effectiveness and to minimize compaction, cultivate late in the irrigation cycle when soils are relatively dry. Fields just cultivated should not be irrigated for 12 to 24 hours to allow for complete desiccation of the uprooted weeds.

Herbicides. In Tule Lake and other potato regions with high organic matter content soils, metolachlor (Dual Magnum II) can be applied to the soil surface before potato emergence for control of nightshade, yellow nutsedge, and other weeds. It is preferable to incorporate metolachlor by irrigation, but it also can be incorporated by shallow tillage after planting. Dimethenamid (Outlook) can also be applied preemergence on high organic matter soils. In such case, use the high rate (0.98 lb a.i. per acre); application must be followed by rainfall, irrigation or shallow mechanical incorporation. Where soils have more than 0.5% organic matter, a postemergent application of metribuzin (Sencor) or metribuzin/rimsulfuron (Matrix) combination may be used to control emerged broadleaf weeds. Best treatment timing is when potatoes are 4 to 8 inches and weeds have just emerged, but sometimes it is used for salvage control of large weeds in older potatoes. Metribuzin is not recommended on some sensitive potato varieties.

If using sequential applications of EPTC in Kern County, apply it after planting with rolling cultivators and incorporate it into the beds, or apply it by chemigation through solid set sprinklers after the beds are cultivated and shaped. Results are sometimes erratic, especially when rain does not occur and irrigations are not made soon after application to activate it before weeds emerge.

Pendimethalin (Prowl) or trifluralin (Treflan, etc.) usually are not needed if EPTC is used sequentially, but pendimethalin is often used postplant with EPTC to suppress nightshade. Pendimethalin applied at low label rates to the surface before potatoes emerge and then sprinkle irrigated, effectively controls pigweeds, lambsquarters, nightshades, and mustard species with reasonable safety. After potato emergence, the risk of crop injury increases; do not use after potatoes reach 6 inches in height. In varieties that have less top growth, adding a low rate of pendimethalin or trifluralin to the postplant application of EPTC enhances late season grass and pigweed control. On winter plantings, emerged nettle and winter mustards can be controlled with paraquat (Gramoxone Inteon) before potatoes emerge.

WEED MANAGEMENT AT HARVEST

In areas where potato plants die soon after irrigation is withheld, early harvest helps reduce weed populations by destroying weeds before their seeds mature. Alternately, after the vines and weeds have been shredded, a 2- to 6-inch soil mound can be created over the beds to help maintain soil moisture around the potatoes during the skin-hardening period. At the same time, weed germination is prevented because the surface soil is dry.

Herbicides. At-harvest treatment to rolled beds with paraquat, diquat (Reglone Desiccant), or Des-i-cate are options for kill of nutsedge and other weed foliage to stop further growth immediately. Burning with propane is sometimes used, more to prevent tuber infections of early blight than for weed control. It does desiccate weeds with one pass over, but a second pass is required to ignite the vegetation. This can facilitate rapid mechanical harvest and reduce early blight tuber infections to shipping potatoes.

Normally desiccants are not needed in Kern County, especially on chipping varieties; the vines desiccate when irrigation is discontinued. Stem end browning caused by the use of desiccants is a concern of growers but rarely occurs from desiccating potatoes in California.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Weeds
J. Nuñez, UC Cooperative Extension, Kern Co.
H. Carlson, UC Cooperative Extension, Siskiyou Co.
Acknowledgment for contributions to the weeds section:
W. E. Bendixen, UC Cooperative Extension, Santa Barbara Co.

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