Resistance in Squash (Cucurbita sp.) to Maize (Zea mays L.) Herbicides

C. Villanueva-Verduzco, M.J. Díaz-Rodriguez, A. Delgadillo-García,
M. Raya-Hernández, R.E. Albores-González, and E. González-Rodriguez

Departamento de Fitotechnia, Universidad Autónoma Chapingo, Chapingo, Mexico 56230, Mexico

Additional index words. Cucurbita argyrosperma, C. ficifolia, C. moschata, C. pepo, maize herbicide resistance, squash, landraces, intercropping, germplasm

Abstract. Throughout many regions in Mexico, production of squash (Cucurbita sp.) occurs as an intercrop with maize (Zea mays). Continued ability to produce squash in these cropping systems is, however, threatened as farmers are increasingly using maize-specific chemical herbicides to which squash is susceptible. A series of greenhouse assays were developed involving three herbicides used for weed control in maizeatrazine, linuron, and simazinefor defining concentrations that show genetic differences for resistance to the herbicides within four species of squash (C. argyrosperma, C. ficifolia, C. moschata, and C. pepo). Assay I (1997) included three separate experiments with C. pepo and the maize herbicides, each applied separately over a range of three to five rates: atrazine (250 to 1250 g of active ingredient (a.i.)/ha, linuron (712 to 950 g a.i./ha) and simazine (1350 to 2700 g a.i./ha). Assays II through VI (1998) corresponded to all four of the squash species, including two botanical varieties of one of the species (C. argyrosperma subsp. argyrosperma vars. argyrosperma and stenosperma). Two experiments were carried out per assay consisting of either atrazine (300 to 700 g a.i./ha) or linuron (631 to 1031 g a.i./ha), applied over five treatment rates. In C. pepo, no plants with resistance to simazine were detected, while none to a few plants (5%) survived to flowering with all treatment rates of atrazine and linuron in Assays I and II, with the exception of the lowest rate in Assay II. Cucurbita pepo and C. moschata exhibited less resistance (0 to 23% plant survivorship) to both atrazine and linuron than did C. ficifolia (2 to 54%) at most treatment rates, while one or both varieties of C. argyrosperma displayed the highest rate of plant survivorship (3 to 63%) to both herbicides over virtually all treatments. The results suggest it may be possible to select squash for resistance to herbicides used in maize production.

Throughout Mexico, many farmers continue to produce squash (Cucurbita sp.) in a cropping system known as the milpa (literally place of maize), in which squash and sometimes other crops (such as beans (Phaseolus vulgaris L.) are intercropped with maize (Zea mays L.). As agricultural modernization occurs in Mexico, changes in cultural practices targeting enhanced production of the main cropmaizemay not necessarily benefit continued production of the other crops in the system. This problem is evident with increased use of maize-selective chemical herbicides to which squash varieties are susceptible. The objective of this study was to establish a basis that will permit selection and development of germplasm of squash that is resistant to preemergent maize herbicides. The study targeted the four species of Cucurbita that are grown widely throughout

Mexico: C. argyrosperma Huber, C. ficifolia Bouché, C. moschata (Duchesce ex Lam.) Duchesce ex Poir., and C. pepo L.

Materials and methods

All squash material used in the assays were local landrace populations from the Chalco-Morelos region of Mexico. Assay I consisted of three experiments with C. pepo conducted in 1997 in the greenhouse at Universidad Autónoma Chapingo, Chapingo, Mexico (Díaz-Rodríguez, 1997). The experiments were carried out in soil containers (diameter and depth: 25 cm) with 10 seeds/container and six replications in a completely randomized experimental design. Seeds were covered with 3 cm of soil. Data on the number of surviving plants were collected. Each experiment consisted of a range of doses of a single herbicide applied at 3¥ to 5¥

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Table 1. Percentage of resistant plants of Cucurbita pepo to different rates of three maize herbicides. Assay I, 1997.

Atrazine Linuron Simazine

Rate Resistant Rate Resistant Rate Resistant

250 0 712 8.0 1350 0

500 1.7 831 0 1850 0

750 0 950 0 2700 0

1000 0 --- --- --- ---

1250 0 --- --- --- ---

zGrams of active ingredient/ha.

Table 2. Percentage of resistant plants of squash (Cucurbita sp.) to different rates of atrazine and linuron. Assays II through VI, 1998.

Cucurbita sp.

C. argyrosperma

var.

Treatmentz C. pepo C. moschata C. ficifolia argyrosperma stenosperma

Atrazine

300 18.7 13.8 22.7 31.1 30.9

400 0.7 7.5 54.3 19.6 70.5

500 0.7 5.4 41.5 2.5 56.8

650 0.5 3.1 17.5 3.4 30.7

700 0.2 1.6 8.4 3.7 4.8

Linuron

631 11.8 23.4 34.7 62.5 63.2

731 4.9 7.3 4.0 50.5 48.4

831 2.1 4.3 2.0 48.8 31.3

931 1.2 1.8 7.9 40.0 20.5

1031 0 1.8 5.9 8.6 7.7

zGrams of active ingredient/ha; four replications ¥ 110 seeds per treatment.

treatment rates. Atrazine was applied at doses 250, 500, 750, 1000, and 1250 g of active ingredient (a.i.)/ha). Linuron was applied at 712, 831, and 950 g of a.i./ha. Simazine was applied at 1350, 1800, and 2700 g a.i./ha.

Assays II through VI were conducted in 1998 at the same site. Peat moss was used as the substrate in containers without drainage. Irrigation was applied and alternated between fresh water and Steiner's nutrient solution. Each assay corresponded to different species or botanical varieties of squash (C. pepo, C. moschata, C. ficifolia, C. argyrosperma var. argyrosperma and C. argyrosperma var. stenosperma (Pangalo) Merrick, respectively). Two experiments were carried out per assay, one with atrazine applied at five different rates (300, 400, 500, 600, and 700 g a.i./ha) and the other with linuron applied at five different rates (631, 731, 831, 931, and 1031 g a.i./ha). Germination percentage, plant height, phytotoxicity, and plant survivorship data were collected.

Results and discussion

Results of plant survivorship are presented here. Table 1 shows results of Assay I (Díaz-Rodríguez, 1997). A small percentage of plants were resistant to the second to lowest treatment rate (500 g a.i./ha) of atrazine (1.7% plant survivorship) and lowest rate (712 g a.i./ha) of linuron (8%), but no resistant plants were observed at other treatment rates for those herbicides. No plants resistant to simazine were found.

Cucurbita pepo exhibited less resistance to atrazine and linuron than the three other species, whereas C. argyrosperma vars. argyrosperma and stenosperma showed the best resistance at most rates to both herbicides (Table 2). Cucurbita moschata exhibited an intermediate level of resistance to both herbicides, while C. ficifolia had better resistance to atrazine than linuron. All species showed low to moderately high degrees of resistance (1.2% to 50.5% plant

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survivorship) to commercially recommended doses of linuron (713 to 950 g a.i./ha), but more limited resistance (0.2% to 8.4%) at about half to one-third of the commercially recommended rate of atrazine (1350 to 2150 g a.i./ha). The results suggest that it may be possible to select populations of squash (Cucurbita sp.) resistant to herbicides used in maize, potentially allowing continued production of squash as an inter

crop with maize where maize-selective herbicides are applied.

Literature cited

Díaz-Rodríguez, M.J. 1997. Evaluación en calabaza (Cucurbita pepo) de herbicidas empleados en maíz. Unpubl. undergraduate thesis. Universidad Autónoma Chapingo, México.

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