Steven A. Fennimore

The Challenges of Specialty Crop Weed Control, Future Directions

The process of labeling new herbicides for specialty crops has always been difficult. Progress in solving specialty crop weed control problems will likely be more challenging in the future. Major crops like corn, cotton, rice, soybean, and wheat are planted on millions of hectares, and most of these crops are treated with herbicides. In contrast, specialty crops (i.e., minor crops, e.g., container ornamentals or lettuce) are planted on 122,000 ha or less; thus, the potential value of herbicide sales is limited in these crops by the low number of hectares planted per crop. High crop value, small hectarage per crop, and generally marginal herbicide selectivity results in a high potential of liability for herbicide registrants and little incentive to label herbicides in these crops. The Interregional Project Number 4 (IR-4) program facilitates the registrations of pesticides on minor crops. Work needed to support pesticide tolerance in a given crop is conducted by IR-4 and cooperators. However, to develop new crop tolerances, the IR-4 process requires new herbicides. The success of glyphosate-resistant soybean has resulted in a less profitable herbicide market for all crops. In response, most primary pesticide manufacturers have reduced the size, or even eliminated herbicide discovery programs. As private industry slows or stops herbicide development, there will be fewer new minor-crop herbicides. Many questions face minor-crop weed scientists. For example, what are other practical solutions to control weeds in minor crops besides herbicides? Should research focus on development of competition models and decision thresholds or on weed removal tools such as robotics? What funding sources are available for minor-crop weed scientists? Are grant programs at the Federal level prepared to increase support for minor-crop weed research? Will university administrators replace retiring specialty crop weed scientists, knowing that their funding sources will produce little overhead? These questions require a response from all parties interested in specialty crop weed control. Nomenclature: Corn, Zea mays L., cotton, Gossypium hirsutum L., lettuce, Lactuca sativa L., rice, Oryza sativa L., soybean, Glycine max (L.) Merr.; wheat, Triticum aestivum L.

Multispectral Machine Vision Identification of Lettuce and Weed Seedlings for Automated Weed Control

Multispectral images of leaf reflectance in the visible and near infrared region from 384 to 810 nm were used to establish the feasibility of developing a site-specific classifier to distinguish lettuce plants from weeds in California direct-seeded lettuce fields. An average crop vs. weed classification accuracy of 90.3% was obtained in a study of over 7,000 individual spectra representing 150 plants. The classifier utilized reflectance values from a small spatial area (3 mm diameter) of the leaf in order to allow the method to be robust to occlusion and to eliminate the need to identify leaf boundaries for shape-based machine vision recognition. Reflectance spectra were collected in the field using equipment suitable for real-time operation as a weed sensor in an autonomous system for automated weed control. Nomenclature: Lettuce, Lactuca sativa L. ‘Capitata’ and ‘Crispa’

Evaluation of Integrated Practices for Common Purslane (Portulaca oleracea) Management in Lettuce (Lactuca sativa) 1

Studies were conducted to develop a weed management strategy for common purslane in lettuce based on available cultural and chemical control methods and an understanding of common purslane biology. A wide-band application of bensulide or pronamide reduced common purslane emergence compared with the standard narrow band. Both narrow and wide bands of pronamide as well as a wide band of bensulide reduced the time required to thin and hoe lettuce. The wide-band pronamide treatment reduced thinning time more than did the narrow band. Overall, none of the herbicide weed management expenses were lower for pronamide treatments than for bensulide and the control. Wide-band bensulide reduced weed management expenses compared with the control, whereas narrow bands did not. Common purslane plants uprooted 1 or 2 wk after emergence (WAE) did not produce any viable seed. Plants uprooted 3 WAE produced from 1 to 60 viable seeds, and seed production increased rapidly from 4 to 6 WAE. Flame and 2% (v/v) glyphosate treatments reduced seed production by uprooted common purslane. Nomenclature: Bensulide; pronamide; common purslane, Portulaca oleracea L. #3 POROL; lettuce, Lactuca sativa L. Additional index words: Banded herbicide, weed seed production. Abbreviations: DAT, days after treatment; GDD, growing degree day; WAE, weeks after emergence.

Evaluation and Economics of a Machine-Vision Guided Cultivation Program in Broccoli and Lettuce

Abstract Machine-vision cultivator guidance systems are commercially available to growers, but little work has been done to determine if these guidance systems can improve integrated weed management systems in vegetable crops. Studies were conducted in 2005 and 2006 in broccoli and lettuce to evaluate band-applied DCPA or pronamide, respectively, and four noncultivated bands ranging from 5.1 to 12.7 cm. DCPA or pronamide were applied in bands centered on the seed line at 0, 7.6 or 12.7 cm wide. A commercial machine-vision system was used to guide a commercial cultivator. Generally, weed densities and hand-weeding times were less where the DCPA band in broccoli or the pronamide band in lettuce were 7.6 or 12.7 cm wide compared to no herbicide. Weed densities were lowest in both crops where the noncultivated band width was 5.1 cm compared to 12.7-cm noncultivated bands. For broccoli in both 2005 and 2006, net returns above production costs were generally higher in the 7.6- and 12.7-cm-wide DCPA bands compared with the no-herbicide band. In lettuce in both years, the no-pronamide treatment had higher net returns, when compared with the 7.6- and 12.7-cm pronamide bands. Lettuce yields and higher net returns in the no-pronamide treatment compared to the 7.6- and 12.7-cm pronamide bands may be due to slight yield reduction from pronamide. Results suggest that pronamide was not needed during the dry months of the year when weed management tools such as hand-weeding and cultivation work very well. However, in periods of rainy weather when cultivation and hand-weeding are not possible, then pronamide would likely provide the greatest economic benefit. Given the large impact of cultivation on vegetable weed management programs, the greatest potential benefit of machine-vision guided cultivators is if they facilitate more timely and effective cultivation. Nomenclature: DCPA; pronamide; broccoli, Brassica oleracea L. var. botrytus L. ‘Marathon’; lettuce, Lactuca sativa L. ‘Sniper’, ‘PIC 714’, and ‘Darkland’

Evaluation and Economics of a Rotating Cultivator in Bok Choy, Celery, Lettuce, and Radicchio

Abstract A commercial intrarow rotating cultivator was tested for weed removal and impact on hand-weeding times in bok choy, celery, lettuce, and radicchio. The rotating cultivator was tested as an automated crop thinner and weeder in direct-seeded bok choy and lettuce as an alternative to hand-thinning and -weeding. The rotating cultivator utilized machine-vision guidance to align a rotating disk with the crop plant to be saved and to remove weeds and undesired crop plants. The rotating cultivator was compared to a standard interrow cultivator, which could not remove weeds from the plant line. Main plots were cultivator type, rotating, or standard, and subplots were herbicides: pronamide for lettuce or prometryn for celery. Weed densities, hand-weeding times, crop stand, and yields were monitored. Economic analysis was performed on a subset of the data. The intrarow rotating cultivator was generally more effective than the standard interrow cultivator for reducing weed densities and hand-weeding times. However, the rotating cultivator reduced seeded lettuce stands by 22 to 28% when compared to hand-thinning and standard cultivation, resulting in lower yields and net returns. In transplanted celery, lettuce, and radicchio, the rotating cultivator removed more weeds than the standard cultivator, and reduced stands by just 6 to 9% when compared to the standard cultivator. In transplanted lettuce, the rotating cultivator was more precise and did less damage to the crop. Because transplanted crops were larger than the weeds, they were more easily differentiated using this technology. Net returns were therefore similar between the two cultivators. What is needed for celery and leafy vegetables is an effective intrarow weed removal system that reduces or eliminates the need for hand-weeding yet does not reduce yields. The rotating cultivator was developed for transplanted crops, where it performs adequately, but it cannot be recommended in the seeded crops evaluated. Nomenclature: Prometryn; pronamide; bok choy; Brassica rapa L. subsp. chinensis; celery; Apium graveolens L. var. dulce (Mill.) Pers.; lettuce; Latuca sativa L.; radicchio; Cichorium intybus L. Resumen Se evaluó el uso de un cultivador comercial rotativo intra-línea para la eliminación de malezas y su impacto en la deshierba manual en bok choy, apio, lechuga y radicchio. El cultivador rotativo fue evaluado para ralear el cultivo y para deshierbar automáticamente en bok choy y lechuga de siembra directa como alternativa al raleo y a la deshierba manual. El cultivador rotativo fue guiado con una máquina de visión para alinear un disco rotativo con la planta del cultivo que debía ser salvada y para remover las malezas y las plantas del cultivo no deseadas. El cultivador rotativo fue comparado con un cultivador inter-línea estándar, el cual no podía remover malezas de la línea de siembra. Las parcelas principales fueron el tipo de cultivador, rotativo o estándar, y las sub-parcelas fueron herbicidas: pronamide para la lechuga o prometryn para el apio. Se determinó las densidades de malezas, los tiempos de deshierba manual, y los rendimientos. Se realizó un análisis económico en un subgrupo de los datos. El cultivador rotativo intra-línea fue generalmente más efectivo que el cultivador inter-línea estándar para reducir las densidades de malezas y los tiempos de deshierba manual. Sin embargo, el cultivador rotativo redujo el número de plantas de lechuga establecidas en siembra directa en 22 a 28% en comparación con el raleo manual y el cultivador estándar, lo que resultó en menores rendimientos e ingresos netos. En apio, lechuga y radicchio de trasplante, el cultivador rotativo removió más malezas que el cultivador estándar, y redujo el número de plantas establecidas en solamente 6 a 9% cuando se comparó con el cultivador estándar. En la lechuga de trasplante el cultivador rotativo fue más preciso y causó menos daño al cultivo. Los cultivos trasplantados fueron más fácilmente diferenciados usando esta tecnología porque fueron más grandes que las malezas. Los ingresos netos fueron de esta forma, similares entre los cultivadores. Lo que se necesita para el apio, y los vegetales de hoja es un sistema efectivo para la remoción de malezas intra-línea que reduzca o elimine la necesidad de deshierba manual sin reducir los rendimientos. El cultivador rotativo fue desarrollado para cultivos de trasplante, en los cuales se desempeña adecuadamente, pero este no puede ser recomendado para cultivos de siembra directa.

Mustard and Other Cover Crop Effects Vary on Lettuce Drop Caused by Sclerotinia minor and on Weeds

Mustard cover crops have been suggested as a potential biofumigant for managing soilborne agricultural pests and weeds. We conducted several experiments in commercial lettuce fields in the Salinas Valley, CA, to evaluate the effects of mustard cover crops on lettuce drop caused by Sclerotinia minor and on weed density and seed viability. In a long-term study, we measured the effects of white and Indian mustard cover crops on the density of S. minor sclerotia in soil, lettuce drop incidence, weed densities, weed seed viability, and crop yield in head lettuce. We also tested broccoli and rye cover crop treatments and a fallow control. Across several short-term studies, we evaluated the density of S. minor sclerotia in soil, lettuce drop incidence, weed densities, and weed seed viability following cover cropping with a mustard species blend. Numbers of sclerotia in soil were low in most experimental locations and were not affected by cover cropping. Mustard cover crops did not reduce disease incidence in the long-term experiment but the incidence of lettuce drop was lower in mustard-cover-cropped plots across the short-term experiments. With the exception of common purslane and hairy nightshade, weed densities and weed seed viability were not significantly reduced by cover cropping with mustard. Head lettuce yield was significantly higher in mustard-cover-cropped plots compared with a fallow control. Glucosinolate content in the two mustard species was similar to those measured in other studies but, when converted to an equivalent of a commercial fumigant, the concentrations were much lower than the labeled rate for lettuce production. Although mustard cover cropping resulted in yield benefits in this study, there was little to no disease or weed suppression.

Weed Control in Glyphosate-Tolerant Lettuce (Lactuca sativa)1

Field studies were conducted in Arizona and California to evaluate the performance of glyphosate-tolerant lettuce and to determine the critical time of weed removal. Glyphosate was applied as a single or as a sequential application at 840 g ae/ha. Single glyphosate applications were made to lettuce at the two-, four-, six-, and eight-leaf stages. Sequential applications were made to lettuce at the two- or four-leaf stage followed by (fb) a second application 14 d after the first. Weed control efficacy, weeding times, and lettuce yield were all measured. Overall, glyphosate applied postemergence (POST) provided better weed control than the commercial standards bensulide or pronamide applied preemergence. Single glyphosate applications at the four-leaf stage and sequential applications at the two-leaf stage fb a second application 14 d later provided excellent control of most weeds, including redroot pigweed. Estimates of the critical time of weed removal were 26 to 29 d after emergence. Glyphosate treatments caused no adverse effects on lettuce. Lettuce head fresh weights in the glyphosate treatments were equal to or higher than those in bensulide or pronamide treatments. For crops such as lettuce, with few effective herbicides, the development of glyphosate-tolerant lettuce offers the opportunity to develop effective POST weed control programs. Nomenclature: Bensulide; glyphosate; pronamide; redroot pigweed, Amaranthus retroflexus L. #3 AMARE; lettuce, Lactuca sativa L. ‘RR Raider 323’. Additional index words: CAPBP, Capsella bursa-pastoris, CHEAL, CHEMU, Chenopodium album, Chenopodium murale, ECHCO, Echinochloa colonum, glyphosate-tolerant lettuce, herbicide tolerance, iceberg lettuce, LEFUN, Leptochloa uninerva, MALPA, Malva parviflora, POROL, Portulaca oleracea, Solanum sarrachoides, SOLSA, time-of-application. Abbreviations: DAE, days after emergence; fb, followed by; POST, postemergence.

Integration of Oxyfluorfen into Strawberry (Fragaria×ananassa) Weed Management Programs

Abstract Field trials were conducted at three California locations near Oxnard, Salinas, and Watsonville from 2002 to 2006 to evaluate broadleaf weed control and tolerance of strawberry to oxyfluorfen. Oxyfluorfen applied at 0.3 and 0.6 kg/ha before strawberry transplanting reduced densities of broadleaf weeds such as California burclover, hairy nightshade, little mallow, shepherds-purse, and yellow sweetclover 70 to 100% compared with nontreated plots but did not control horseweed. Oxyfluorfen application resulted in 9% and 19% greater visible injury to strawberry for the two rates, respectively, compared with nontreated plants in 1 yr but did not reduce strawberry yield. After oxyfluorfen application at 0.6 kg/ha, strawberry plants had 5 to 48% more injury than nontreated plants in subsequent years but early-season yields were similar. Hand-weeding time was reduced 30 to 50% compared with nontreated plots regardless of oxyfluorfen rate. Both water-based and solvent-carrier formulations of oxyfluorfen resulted in similar weed control, strawberry injury, and fruit yield. Plastic mulch installation after oxyfluorfen application but before planting reduced injury to strawberry more than 50% compared with nonmulched beds. Oxyfluorfen applied 30 d before strawberry transplanting had similar crop injury and yield to applications made 15 and 7 d before planting. These results suggest that oxyfluorfen can be used safely in California plasticulture strawberry production for control of common weed species and to reduce labor inputs associated with hand weeding. Nomenclature: Oxyfluorfen; California burclover, Medicago polymorpha L. MEDPO;hairy nightshade, Solanum physalifolium Rusby SOLSA; horseweed, Conyza canadensis L. ERICA; little mallow, Malva parviflora L. MALPA; shepherds-purse, Capsella bursa-pastoris L. CAPBP; yellow sweetclover, Melilotus officinalis L. MEUOF;strawberry, Fragaria×ananassa

Technology for Automation of Weed Control in Specialty Crops

Specialty crops, like flowers, herbs, and vegetables, generally do not have an adequate spectrum of herbicide chemistries to control weeds and have been dependent on hand weeding to achieve commercially acceptable weed control. However, labor shortages have led to higher costs for hand weeding. There is a need to develop labor-saving technologies for weed control in specialty crops if production costs are to be contained. Machine vision technology, together with data processors, have been developed to enable commercial machines to recognize crop row patterns and control automated devices that perform tasks such as removal of intrarow weeds, as well as to thin crops to desired stands. The commercial machine vision systems depend upon a size difference between the crops and weeds and/or the regular crop row pattern to enable the system to recognize crop plants and control surrounding weeds. However, where weeds are large or the weed population is very dense, then current machine vision systems cannot effectively differentiate weeds from crops. Commercially available automated weeders and thinners today depend upon cultivators or directed sprayers to control weeds. Weed control actuators on future models may use abrasion with sand blown in an air stream or heating with flaming devices to kill weeds. Future weed control strategies will likely require adaptation of the crops to automated weed removal equipment. One example would be changes in crop row patterns and spacing to facilitate cultivation in two directions. Chemical company consolidation continues to reduce the number of companies searching for new herbicides; increasing costs to develop new herbicides and price competition from existing products suggest that the downward trend in new herbicide development will continue. In contrast, automated weed removal equipment continues to improve and become more effective.

Herbicide Evaluation for Fresh Market Celery

Field studies were conducted near Oxnard, CA and in two locations near Salinas, CA in 2002 and 2003 to evaluate efficacy and safety of six herbicides in celery. s-metolachlor at 0.6 and 0.7 kg ai/ha and flufenacet at 0.5, 0.6, and 0.7 kg ai/ha PRE were safe to celery and controlled 81 to 94% of yellow nutsedge at densities < 20 plants/m2. At yellow nutsedge densities > 20 plants/m2, the most efficacious treatments were 1.1 kg/ha of s-metolachlor or 0.7 kg/ha of flufenacet, which reduced nutsedge densities by 71 or 53%, respectively; however, both resulted in slight injury to celery. All other herbicides generally did not injure celery and none reduced marketable crop yield. Flumioxazin pretransplant (PRE) at 0.1 and 0.2 kg ai/ha controlled broadleaf weeds near 100% at all locations, suggesting that it can be an effective alternative to standard linuron and prometryn, which are applied post-transplant (POST) in celery. Nomenclature: S-metolachlor, flufenacet, flumioxazin, prometryn, linuron, oxyfluorfen, yellow nutsedge, Cyperus esculentus L. CYPES, celery, Apium graveolens L