Developing a weed management program in conventional pear orchards is a challenge, varies from orchard to orchard and is influenced by weed species populations, weed pressure, management practices and local environmental condition. Integrated weed strategies ideally involve the use of multiple strategies, including mowing, chemically mowing, discing and cultivation in the row middles of trees, herbicide strip sprays and the adoption of selected cover crops.
Pear orchards usually require intensive irrigation and high moisture in the soil, especially in late May, June and early July, when the fruit is increasing in size. Optimum soil moisture and temperature in the orchard floor favors a high pressure of summer weed infestation, which requires multiple post-emergent herbicide applications to keep the pear orchard floor weed-free during the growing and harvesting season.
For California pear orchards, recommended herbicide programs may include a fall/winter (November to February) strip spray with preemergent (indaziflam, rimsulfuron, pendimethalin, or flumioxazin) in a tank mixture with post-emergent (glyphosate or saflufenacil). However, during the growing season, the strip spray herbicide programs are primarily performed using only post-emergent herbicides in April, late May and July.
Improve Herbicide Program Rotation
Over the past few decades, glyphosate has been the most used herbicide registered for post-emergence non-selective weed management of annual and perennial weeds in conventional pear orchards. However, some weed populations, such as Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), annual bluegrass (Poa annua) horseweed (Conyza canadensis) and hairy fleabane (Conyza bonariensis), have developed resistance to glyphosate, and poor control of weeds with glyphosate application programs have been observed more frequently in California orchards.
Practitioners are seeking broad-spectrum herbicide alternatives to glyphosate. However, substitutes have scarcely been evaluated due to glyphosate’s history of being effective and affordable. Although this doesn’t seem imminent, if glyphosate were no longer available, glufosinate-ammonium appears to be the most efficient and economical option. Glufosinate is a contact non-selective herbicide that is very effective against annual broadleaf and grass weeds but is less effective against biennial and perennial weeds and may require sequential applications to achieve satisfactory control. Overall, the labels of glyphosate and glufosinate indicated outstanding performance in controlling a wide variety of weeds. For this reason, glufosinate has been ranked as the best alternative currently available.
Consider Adding Preemergent Herbicides in Late Spring Spray Programs
Weed infestation in pear orchards is a year-round problem, especially during the growing season and preharvest with various annual and perennial species such as field bindweed (Convolvulus arvensis), nutsedge (Cyperus spp.) and summer grass such as Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), foxtails (Setaria spp.), crabgrass (Digitaria sanguinalis), common bermudagrass (Cynodon dactylon), etc. Due to high weed infestation during the growing season, post-emergent-only herbicide programs have short-lasting weed control (Fig. 1), requiring multiple sequential applications to keep weeds below an acceptable threshold. Beyond that, the use of many herbicide programs may require minimum intervals between the last herbicide application and harvest of up to 75 days for pear growers delivering the crop to specific markets. Therefore, it is difficult to select herbicide application programs that provide long-lasting control.
Generally, preemergent herbicide programs in pear orchards are typically applied in the fall/winter to early spring and have scarcely been evaluated for late spring application. This lack of information warrants more research regarding the effectiveness and crop safety of preemergent and post-emergent herbicide programs for late spring application. We believe preemergent herbicide programs added to the tank mixture may bring several benefits for late spring application, such as improved long-lasting control, prevention of herbicide-resistant weed evolution, reduction in total number of operations required for weed management and increase in preharvest minimum intervals.
Procedures
In late spring 2024, we established two herbicide field trials in Lake and Mendocino counties to compare the effectiveness of glyphosate and glufosinate sprayed side by side and to evaluate the advantages and disadvantages of (preemergent and post-emergent) herbicide programs with different modes of action to improve weed management in pear orchards.
For these studies, we compared glyphosate (Roundup PowerMAX®) at 64 fl oz/ac and glufosinate (Rely® 280) at 56 fl oz/ac applied alone and in a tank mixture at two different rates with indaziflam (Alion®) at 3.5 and 4.5 fl oz/ac, rimsulfuron (Matrix®) at 3.0 and 4.0 fl oz/ac, or pendimethalin (Prowl H2O®) at 70.4 and 102.4 fl oz/ac, applied in a water carrier volume of 30 gallons per acre (GPA) with 11003 VS flat-fan spray nozzles. To reduce costs, growers may consider generic herbicides rather than the brand-name counterparts used in these trials. In many cases, generic herbicides may have the same effectiveness as brand-name counterparts at a lower cost (consult your local UCCE farm advisor or your PCA and always read the pesticide label.)
Efficacy
The main weeds present at the trial sites were jungle rice, crabgrass, common bermudagrass, yellow nutsedge and field bindweed. Our results showed both glyphosate and glufosinate applied in late May provided excellent weed control greater than 90% for most weeds present at the trial sites. Overall, glufosinate proved to be a broad-spectrum herbicide with equivalent grass and broadleaf weed control to glyphosate (Fig. 2). However, both glyphosate and glufosinate have low residual activity, and most weeds began to germinate or regrow from regenerative underground propagules via roots, rhizomes or tubers, indicating the need for sequential application around four to six weeks after initial application for effective long-term weed control (Fig. 3).
Overall, the results showed glyphosate or glufosinate applied in a tank mixture with the preemergent herbicides indaziflam, rimsulfuron or pendimethalin at the rates used in these studies provided excellent weed control for most weeds present in the orchard sites and improved long-lasting weed control when compared to glyphosate or glufosinate applied alone.
The tank mixture program with preemergent herbicides indaziflam or pendimethalin provided better long-term control of jungle rice and crabgrass than rimsulfuron. On the other hand, all preemergent herbicides showed similar control of broadleaf weeds present in the orchard sites.
Our results also indicated glyphosate or glufosinate applied in a tank mixture with pendimethalin provide inferior control of yellow nutsedge compared to indaziflam or rimsulfuron tank mix (Fig. 4).
In general, late spring application of glyphosate or glufosinate alone and in a tank mixture with indaziflam, rimsulfuron or pendimethalin at the rates used in these studies were safe for pear trees with no injury observed.
Our results indicated glufosinate may be a great alternative to glyphosate with similar efficacy for controlling a broad spectrum of weeds, and adding preemergent herbicide to the late spring herbicide application programs may improve long-term weed control.
The results of these studies and the literature review strongly suggest developing more efficient herbicide application programs or alternatives to glyphosate by using herbicides with different modes of action may help to reduce potentially resistant weeds. Also, adopting spray programs with preemergent and post-emergent herbicides promotes longer-lasting weed control, reducing the number of herbicide applications, increasing the time window between the last herbicide application and the harvest season, and reducing the risk of herbicide residue in the crop.
These studies may contribute to growers and PCAs developing a more complete integrated weed management program in conventional pear orchard systems and potentially result in a reduction of costs by not adopting preharvest strip sprays.
These studies will be repeated in 2025 to confirm preliminary data assessed in 2024. The mention of active ingredients or products in this article is not an endorsement or recommendation. Consult your local UCCE farm advisor or your PCA for a recommendation and always read the pesticide label; the label is the law.
The authors would like to thank California Pear Advisory Board and Pear Pest Management Research Fund for funding these studies. We thank Wilfredo Bello, UCCE agricultural technician in Lake County, for the technical support.
