Southwest Florida receives about 50 inches of rainfall annually. Who would have thought that a dry spell - an almost totally rainless period - could last more than 40 days in this kind of climate? But, that’s what happened during Jan. and Feb., 2005 in Immokalee. As a result of this dry weather, newly transplanted seedlings were exposed to glyphosate herbicide.

Tomatoes were planted during fall of 2004 at the Southwest Florida Research and Education Center as part of an ongoing research project. When the tomato experiment was completed, vines were sprayed with glyphosate. The plan was to double crop the field with a second crop - watermelon. Tomato vines were not killed with paraquat herbicide, a normal practice when double cropping, because it was decided the crown of the plants could sprout new growth and cause problems later in the season. Vines were sprayed with glyphosate on Jan. 19 and allowed to die. More than 40 days later, 'Tri-X 313' and 'Mardi Gras' watermelon seedlings were transplanted into the same field using the same plastic mulch (Fig. 1).  After transplant, a water wagon was sent through the field to help transplants get off to a good start.

In less than two weeks, it became apparent that some transplants were exhibiting injury symptoms (Figs. 2 and 3). Young, developing leaves on these transplants were pale yellow, while older leaves remained dark green. Symptoms were similar to those of tomato and pepper receiving sublethal doses of glyphosate (Gilreath et al., 2000; Romanowski, 1980; Russo, 1990).

Some of the young leaves developed intervenal chlorosis (Fig. 4).  Injured plants were randomly scattered throughout the 1.5 acre field, and replacement plants were transplanted next to injured plants (Fig. 5).  Some of the original transplants did not grow out of the injury (Fig. 6), and the number of plants exhibiting symptoms resulted in a lack of uniformity in the field.  Finally, it was decided to replant the entire field.

When rainfall is known to be lacking in the weeks after glyphosate application, common growers practice in the area is to apply water to the surface of the plastic with a water wagon. Florida Agricultural Weather Network (FAWN) weather data clearly showed our location lacked adequate rainfall to wash residues from the plastic (Table 1), and the herbicide should have been manually washed from the plastic before planting.

Many pesticides quickly photodegrade in sunlight.  Glyphosate does not, and glyphosate products have labels that warn about this problem. The following excerpt is from the label of Roundup Ultra Max [Monsanto Co., active ingredient glyphosate, N-(phosphonomethyl)glycine, in the form of its isopropylamine salt]:

“PRECAUTIONS, RESTRICTIONS: When applying this product prior to transplanting or direct-seeding crops into plastic mulch, care must be taken to remove residues of this product, which could cause crop injury, from the plastic prior to planting. Residues can be removed by a single 0.5-inch application of water, either by natural rainfall or via a sprinkler irrigation system. Care should be taken to insure that the wash water flushes off the plastic mulch and does not enter transplant holes. Applications made at emergence will result in injury or death to emerged seedlings.”

Exposure to the product occurs several different ways. Transplant trays, when placed directly on top of the plastic mulch, expose roots of the transplants to glyphosate salts on the plastic. Trays are sometimes slid down the row as plants are transplanted, thus further exposing roots to glyphosate residue.

In addition, glyphosate on the plastic is solubilized when plants are watered with a water wagon or when a first rainfall occurs.  Plant foliage then comes in direct contact with glyphosate in solution on the plastic. Glyphosate can also be washed directly into the planting hole where it may affect plant growth (Cornish, 1992).

In conclusion, be sure to monitor rainfall totals during the period of time after glyphosate application and before transplant.

References

Cornish, P.S. 1992. Glyphosate residues in a sandy soil affect tomato transplants. Austr. J. Expt. Agric. 32:395-399.

Gilreath, J.P. C.A. Chase, S.J. Locascio. 2000. Phytotoxic effects of glyphosate on pepper (Capsicum annuum). Weed Technology 14:488-494.

Romanowski, R.R. 1980. Simulated drift studies with herbicides on field-grown tomato. HortScience 15:793-794.

Russo, V.M. 1990. Reaction of tomato cultivars to a sublethal dose of glyphosate. HortScience 25:1662.

Table 1. Daily totals of rainfall during a 43-day period in Immokalee. Data from the FAWN station located at SWFREC.
Month (2005)	Day	Rainfall (in)	Field operation
January	19	0	glyphosate applied
	20	0
	21	0
	22	0
	23	0.14
	24	0
	25	0
	26	0
	27	0
	28	0
	29	0
	30	0
	31	0
February	1	0
	2	0
	3	0
	4	0
	5	0
	6	0
	7	0
	8	0
	9	0
	10	0
	11	0
	12	0
	13	0
	14	0
	15	0
	16	0
	17	0
	18	0
	19	0
	20	0
	21	0
	22	0	Seedlings transplanted

Kent Cushman
Vegetable Specialist, SWFREC
Vegetarian, 05-04