Page 246

 
WISCONSIN  HORTICULTURE 
Experiments With Ground 
  Spraying For Combating Apple Scab 
                                by G. W. Keitt and C. N. Clayton 
F   OR   many  years the chief 
    method of apple scab control 
 has been repeated spraying or 
 dusting with the aim of protect- 
 ing the susceptible parts against 
 infection. While this protectant 
 spraying has brought great ad- 
 vances in scab control, it still has 
 some important defects. It is ex- 
 pensive, laborious, sometimes se- 
 riously injurious to fruit or foli- 
 age, and liable to failure tinder 
 some conditions. One of the most 
 important shortcomings of pro- 
 tectant spraying is that it ordi- 
 narily permits enough leaf infec- 
 tion to enable the scab fungus to 
 overwinter and produce an abund- 
 ant supply of ascospores to start 
 heavy infection the next spring. 
 For several years the Wisconsin 
 Agricultural Experiment Station 
 has been studying possibilities of 
 a direct chemical attack on the 
 fungus at a weak stage in its life- 
 history, with the aim of reducing 
 it to such a low survival that the 
 protectant spraying program can 
 be made less expensive or in- 
 jurious and more certain of suc- 
 cess. 
   Spraying the Overwintered 
     Leaves on the Ground 
  A weak point in the life-history 
of the scab fungus occurs in the 
spring before the first infection 
takes place. Under Wisconsin 
conditions the only imliortant 
overwintering of the fungus oc- 
curs in the dead leaves'. In the 
spring the fungus is, therefore, 
prostrate on the ground. If this 
dest ructive pest were large 
enough to be readily seen, it is 
unlikely that we should let it pass 
this stage unmolested. If it were 
as large as Canada thistle, for 
example, would we allow it to 
survive year after year on the 
floor of our orchards? It is true 
that v a r i o u s recommendations 
have been made for the disposal 
of the overwintered leaves, as by 
raking and burning or turning 
them under, but none of these 
methods have been efficient or 
practicable enough to come into 
general use. 
   For several y e a r s , we have 
 made small-scale tests of the ef- 
 fectiveness of spraying the dead 
 leaves on the ground with vari- 
 otis chemicals to kill the fungus 
 or prevent it from maturing and 
 discharging ascospores. Certain 
 of the preparations tested showed 
 a high degree of effectiveness in 
 limiting ascospore discharge. In 
 the past season, one of these, a 
 l)roprietary preparation known as 
 Elgetol Extra, was tested as a 
 ground spray in a commercial or- 
 chard. 
 The floor of a 6-acre McIntosh 
 orchard in sod near Sturgeon Bay 
 was sprayed shortly before bud- 
 break with Elgetol Extra diluted 
 with water at the rate of 1 gal- 
 lon in 100 gallons. It was applied 
 by means of double-nozzle spray 
 guns at the rate of about 450 gal- 
 lons per acre, care being taken to 
 cover the ground thoroughly and 
 to spray more heavily where the 
 leaves had drifted into heaps. The 
 orchard remained  uncultivated 
 throughout the season. A small 
 McIntosh orchard about three- 
 tenths of a mile away, which re- 
 ceived no ground spray, served 
 as a check. A few trees in these 
 orchards were left without sum- 
 mer spraying for comparative 
 studies on disease development. 
 Certain modified spray programs 
 were used in the orchard that re- 
 ceived the ground spray, in or- 
der to gain evidence as to wheth- 
er the ground spraying aided in 
scab control. Similar experiments 
were   conducted  in  McIntosh, 
Dudley, and Wealthy orchards 
about 4 miles away, in which no 
ground spray was applied. 
             Results 
   Counts of scab infections on 
 fruit and foliage of the unspray- 
 ed trees in the ground-sprayed 
 orchard and the near-by orchard 
 that served as a check indicated 
 that, through the critical period 
 for scab control, extending to 
 about three weeks after petal- 
 fall, the ground spray reduced the 
 severe scab epidemic by about 
 nine-tenths. 
 A program of 7 lime-sulphur 
 treatments in the orchard that re- 
 ceived Elgetol gave 1 per cent 
 of scabbed fruit at harvest, 
 whereas a similar 8-spray pro- 
 gram in the experimental block 
 of McIntosh about 4 miles away, 
 which did not receive any ground 
 treatment, gave 32 per cent. A 
 4-spray program in the Elgetol- 
 treated orchard gave only 15 per 
 cent scab. It is recognized that 
 other factors than the Elgetol 
 treatment may have played a part 
 in producing the differences in 
 results in the treated and un- 
 treated orchards, but the Elgetol 
 spray is thought to have been 
 the chief factor concerned. 
 The Elgetol treatment as ap- 
 plied in these experiments was 
 highly effective in killing the 
fruiting structures (perithecia) of 
the scab fungus in the overwin- 
tered leaves that were wet by the 
spray. However, in leaves pro- 
tected in the lower parts of 
the drifted heaps, perithecia es- 
caped injury. Under the condi- 
tions encountered, this seems not 
to have prevented a high degree 
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246 
May, 1940