WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



85 



and grasses in the cotton row. It requires the ap- 

 plication of a carefully controlled flame to the drill 

 row of cotton plants at a rate that does not injure 

 the crop while killing the more delicate weeds. 



Chemicals can be used to artificially stimulate 

 seed germination. Such stimulation has the poten- 

 tial of providing an even-aged weed population 

 that is less difficult to control. Recently, two un- 

 known germination stimulants of witchweed were 

 isolated from cotton roots. An unidentified alkali- 

 labile corn root exudate was also found that would 

 stimulate germination of witchweed. "Witchweed 

 is an exceedingly serious pest along the middle 

 Atla ntic seaboard. 



The biological control of plants was firmly es- 

 tablished in the thirties following the introduction 

 of a moth, Cactoolastis cactorum, from Argentina 

 into Australia for the control of pear cactus in that 

 country. The snail, Marisa comuarietis and two 

 fish genera have since shown promise for the con- 

 trol of submersed water weeds. 



The control of Klamath weed {Hypericum per- 

 foratum) in northern California, Oregon, and 

 Washington by beetles of the genus Ghrysolina is 

 a good example of biological control of weeds. By 

 1944. over 2 million acres of California rangeland 

 were infested with this weed. Release of the beetles 

 in California began in 1945. As soon as beetles be- 

 came abundant, collecting programs were orga- 

 nized at county levels so that ranches could assist 

 in distributing the insects. In 4 years it became evi- 

 dent that good control was being obtained. In 7 

 years, the weed was under control. In 1956, 

 Klamath weed was removed from the list of nox- 

 ious weeds in California. 



Fungicides 



Persistence in the Environment. — The adverse 

 effects of fungicides upon our environment are not 

 widespread. Where harmful effects have been 

 noied or suspected, the heavy metal component of 

 the fungicide was usually involved. Many fungi- 

 cides contain copper, zinc, mercury, iron, or man- 

 ganese. The literature pertaining to the persistence 

 of these metals in soils and their adverse effects 

 on plants has recently been reviewed. 9 



Apple orchards in England with a long copper- 

 spray record were found to contain in the surface 



» LAGERWERFF, ,T. V. HEAVY-METAL contamination of soil. 



;•>(! hefore the Amer. Assoc. Adv. Scl. Meetings, Washing- 

 ton. D.C.. Dec. 29, 1956. 



mat more than 1,500 p.p.m. of copper. The surface 

 soil in old vineyards in France was found to con- 

 tain up to 845 p.p.m. of copper. In sandy soils used 

 for citrus in Florida, copper concentrations in the 

 surface layer were found to exceed 50 p.p.m. This 

 level was sufficiently high to be toxic to citrus 

 seedlings. Clover and alfalfa plants are particu- 

 larly sensitive to copper in the soil. 



Research has developed copper fungicides that 

 hold the copper in a chelated, less available, form. 

 Zinc and manganese can accumulate to toxic levels 

 in soils, but the evidence indicates that application 

 of fungicides containing these metals is usually not 

 at fault. 



Mercury found in soils usually comes from 

 fungicides. Mercury vapor can be toxic to plants 

 and the effects are usually noted in greenhouses 

 where ventilation is low. 



Research has shown that the unfavorable effects 

 of heavy metals on plants can generally be allevi- 

 ated by (1) increasing the soil pH above 6, such as 

 by liming, (2) adding organic matter to the soil. 

 (3) applying an iron chelate either foliarly or to 

 the soil, and (4) applying phosphatic fertilizers 

 if the^)H is managed properly. 



Alternates to the Use of Fungicides. — Seeking 

 ways and means to alleviate the ravages of plant 

 diseases is as old as the arts of agriculture. 

 Through crop rotation, cultural practices, sanita- 

 tion, and elimination of alternate hosts, many a 

 plant disease is controlled or partially kept in 

 check, thereby reducing or eliminating the need 

 for fungicides. Breeding varieties that are resistant 

 to diseases has been the most effective approach 

 towards controlling disease without resorting to 

 the use of chemical fungicides. 



Brown rot can be exceedingly destructive in a 

 crop of peaches. The sources of infect ion are spores 

 formed by "mummies" — infected peaches that 

 dried on the tree before dropping — that come into 

 direct contact with the soil. Peaches rotting on the 

 ground rarely form mummies. Tims, studies have 

 shown that field sanital ion during pruning or har- 

 vesting should be that of preventing many mum- 

 mies on trees from dropping to the soil . 

 coming sources of in feet ion. 



Bacterial blights of beans can spread very 

 rapidly over a field when the vines are we; front 

 dew ora shower. Cultivating and harvesting damp 



