Desiccation and Defoliation of Plants by Different Herbicides and Mixtures 1

Guava, hibiscus, sorghum, and cotton leaves were treated with 5 herbicides each at 10 dosages. Pentachlorophenol (PCP) and cis−2,3,5,5,5−pentachloro−4−ketopentenoic acid (AP−20) produced the most rapid desiccation at high dosages. The 6,7−dihydrodipyrido(l,2−α:2',I'−c)pyrazidünium salt (diquat) was usually as effective as PCP and AP−20 within 1 to 2 days after treatment, and it produced more extensive desiccation at lower dosages, especially on hibiscus, sorghum, and cotton. Diquat, l,l'−dimethyl− 4,4'−bipyridinium salt (paraquat), dimethylarsinic acid (cacodylic acid), and AP−20 produced complete defoliation of hibiscus within 5 days after treatment but diquat and paraquat were more effective at lower dosages than other defoliants. Diquat completely defoliated hibiscus at 19 µg regardless of placement on the leaf, but higher dosages were required for effective desiccation when applied non−uniformly over the leaf. Effective desiccation with PCP required uniform application to either upper or lower surfaces. Defoliation of leaves after treatment with PCP did not occur. Some herbicides increased the desiccation and defoliation of hibiscus synergistically when added to diquat. Hormone−type herbicides such as 2,4−dichlorophenoxyacetic acid (2,4−D) and 4−amino−3,5, 6−trichloropicolinic acid (picloram) were antagonistic to diquat.