Everglades Station Mimeo Report EES64-4 September 1963
EXPANDING THE ROLE OF HERBICIDES IN CROP PRODUCTION ~
J. R. Orsenigo 2
Herbicides for crop production usually are evaluated according to their direct
effect in weed control and on crop response and yield. The true role of herbicides
is more complex and the potential of these chemicals as indirect adjuncts to crop
production is great. In crops, herbicidal weed control per se may be less important
than indirect benefits. Non-crop herbicide programs may assist effectively in con-
trol of other pests: diseases, insects; nematodes and viruses. This paper will
illustrate briefly some of these relationships which should be useful in expanding
the role of herbicides in production of agronomic and horticultural crops.
Mechanical and cultural:
The time-honored objective of cultivation has been weed control. Effective
herbicide programs can eliminate cultivation in some crops on many soils and prevent
mechanical injury to crop root systems. This avoidance of root injury may be as
important to production as elimination of weed competition.
Herbicide programs should fit crop rotations and cover crop practices lest
cultivated plants become established as weeds in successive plantings. In Venezuela
volunteer rice frequently became a serious weed pest in the sesame crop which fol-
lowed (9a). On Florida's organic soils, sesbania cover crops that mature and seed
may cause serious volunteer infestations in the next crop. Sesbania established
and became a competitive weed in corn treated pre-emergence with 4 lb/A ai of
atrazine or simazine in the Zellwood area (10). Sesbania may be the only weed in
celery treated with CDAA, CDEC or a combination of both in the Belle Glade area(9a)
A classic illustration of disease control through eradication of an alternate
host has been an 18 state program of barberry eradication for prevention of black
stem rust (Puccinia graminis) of certain small grains, particularly wheat. Over a
40-year period, the program cost has been estimated at less than 1 per cent of crop
value and the annual savings have been estimated at some 30 millions of dollars(14).
Here in the South, interesting soil pathogen-weed control relationships have
been investigated in peanuts. Herbicide-treated peanuts have grown more vigorously
than cultivated peanuts. Data are suggestive that particular herbicides (DNBP in
this case) may influence disease development under field conditions (3). Others
reported lessened Sclerotinia stem rot infection and higher yields where an herbi-
cide was used as compared to usual cultivation in which soil moved in wred control
was thrown onto peanut plants. Again, the particular herbicide (DNBP) may have had
some influence on disease incidence apart from weed control and avoidance of
1. This paper was given orally at the 15th Annual Meeting of the Southern Weed
Conference and appears in the mimeographed proceedings of that Conference,
2. Associate Horticulturist, University of Florida Everglades :Eperiment Station,
Belle Glade, Florida.
"dirting" the plants (l4). Other researchers, however, reported possible deleterious
effects of a soil-borne disease complex and an herbicide (sesone) on peanut emer-
gence when the natural crop emergence (no herbicide) was low (1).
Some herbicides, particularly pre-emergence materials, may have direct and
indirect effect on insect populations. In Venezuela, armyworm (Laphycma sp.) damage
in corn plots sprayed pre-emergence with DNBP was nil to slight; often all other
herbicide and control plots were severely damaged to completely denuded by armyworm.
The mechanism of the insecticidal, repellent or alleviating properties of the herbi-
cidal treatment was not developed (16). On the same soils, the control of armyworm
in DNBP-treated rice was less complete than in the corn experiments (9).
Recently in Florida, corn stem weevil (Hyperodes humilis) populations were
reduced greatly in sweet corn sprayed with a pre-emergence herbicide as compared
to those not cultivated during a 24-day insect control program (6). Later experi-
mentation confirmed the value of pre-emergence herbicides in a corn stem weevil
control program (7). The particular herbicides (CDAA and CDEC) used have no known
toxicity to insects. Possible mechanisms for the enhanced control are: removal
of grass and weed growth from the drill thus permitting DDT sprays to effectively
reach the lower corn stem and soil surrounding the plant; non-disturbance or non-
incorporation of DDT deposited on the soil surface; and/or elimination of weed
plants attractive to ovipositing adults.
A field sanitation program with herbicides applied to field borders and non-
cropped areas to eliminate broadleaf weed species apparently reduced populations
of cowpea curculio (Chalcodermus aeneus), melonworm (Diaphania hyalinata) and
pickleworm (D. nitidalis) in southernpea and squash crops in South Florida (5).
Many nematodes have a wide host range among cultivated and wild plant species.
Herbicides have a great potential in indirect control of this crop pest.
In a tobacco-growing region, 25 of 107 weed species were listed as hosts for
the root-knot nematode. Infestation of the four most common weed host species
ranged from 16.1 to 53.8 per cent of plants examined. Ten species in all were
signalled for eradication as a means of reducing root-knot damage to following
The control of weeds and all vegetation has been an important part of the
containment and control program for the burrowing nematode in Florida citrus(8).
Herbicides may be useful in eliminating broadleaf weed species and controlling
root-knot nematodes in kenaf (15).
Certain plant viruses can be controlled by eradication of wild or cultivated
host species. The leaf hopper which transmits curly-top of beets has a feeding
range among cultivated and wild plants of 20 families in which the virus can develop.
Control may be effected by removal of the host species (14).
In South Florida, aphids transmit viruses detrimental to celery, cucumber,
pepper, squash, tomato and watermelon; primary control of these viruses is through
eradication of the wild hosts (11).
A pre-planting program for the control of potato virus Y in pepper and tomato
was initiated in South Florida (13). Isolated farms which had practiced effective
off-season field and fence-row weed control sanitation programs to eradicate night-
shade, Solanum nigrum, had minor levels of potato virus Y in pepper and tomato.
Farms not isolated, or those without effective programs for control of host plants,
experienced earlier and more severe viral infections. On-farm weed control during
the growing season may reduce virus spread through removal of aphid host plants
rather than removal of virus host species (12).
Several examples have illustrated the complex effects herbicides may have in
crop production. The direct benefits from removal of weed competition, reduction
of cultural expense and prevention of cultivation injury may be augmented greatly
by herbicidal practices which influence programs for control of other crop pests.
Both crop and non-crop herbicides can be an indirect key to control of various
plant diseases, insects, nematodes and viruses.
Research and extension personnel should be alert to investigate and develop
these indirect and interdisciplinary relationships opportunely and, through them,
expand the role of herbicides in crop production.
1. Boyle, Lytton W., Ellis W. Hauser and Jack T. Thompson. 1958. The combined
effect of an herbicide and disease on the emergence of peanut seedlings.
2. Bratley, H. E. 1946. Weeds as a factor in the control of root-knot in tobacco
fields. Univ. of Fla. Agr. Exp. Sta. Press Bul. 629.
3. Chappell, W. E. and L. I. Miller. 1956. The effects of certain herbicides
on plant pathogens. P1. Dis. Rptr. 40:52-56.
4. Garren, Kenneth H. and George B. Duke. 1957. The peanut stem rot problem and
a preliminary report on the interrelations of "non-dirting" weed control and
other practices to stem rot and yield of peanuts. P1. Dis. Rptr. 41:424-431.
5. Genung, W. G. 1961. Personal communication.
6. Harris, Emmett D., Jr. and J. R. Orsenigo. 1961. A relationship of chemical
weed control to corn stem weevil control on sweet corn. Proc. Florida State
Hort. Soc. Vol. 74 (In press).
7. ____ and __. 1961. Unpublished
8. Kretchman, D. W. 1962. Controlling weeds in burrowing nematode buffer zones
in Florida citrus groves. Proc. Southern Weed Conf. Vol. 15 (This volume).
9. Orsenigo, J. R. 1956. Rice investigations in Venzuela in 1954. IBEC Research
Institute Bul. 8.
9a. 1954 and 1961. Unpublished data.
10. Scudder, W. T. 1961. Personal communication.
11. Simons, J. N. 1958. Virus diseases affecting vegetables in South Florida.
Proc. Florida State Hort. Soc. 71:31-34.
12. and P. L. Thayer. 1960. State Project 558. Florida
Agr. Exp. Sta. Ann. Rpt. 1960:245.
13. _, J. R. Orsenigo, R. E. Stall and P. L. Thayer. 1959.
Potato virus Y in peppers and tomatoes. Univ. of Florida Everglades Station
Mimeo Rpt. 59-31.
14. Stakman, E. C. and J. George Harrar. 1957. Principles of Plant Pathology.
15. Summers, T. E. 1962. Personal communication.
16. Young, E. F., Jr. and J. R. Orsenigo. 1955. Corn investigations in Venezuela
in 1953 and 1954. IBEC Research Institute Bul. 7.
EES Mimeo 6'--U.