11 



cations, i.e., prepink, pink, cah'x, fourth dust and fifth dust. At the Frost orchard, 

 the trees were dusted four times, no prepink apphcation being used. 



In the Stowe orchard, where check trees >delded 37 per cent scabby fruit, trees 

 dusted with sulfur jdelded 3.7 per cent scabby fruit. At the Sabine orchard, trees 

 dusted with sulfur bore 0.5 per cent scabby fruit, and on the check trees 48 per 

 cent of the fruit was scabby. At the Frost orchard, where no prepink apphcation 

 was used, trees dusted mth sulfur yielded 16.9 per cent scabbj^ fruit, as compared 

 with 84.5 per cent scabby fruit in the check. Sulfur dust controlled apple scab 

 satisfactorily when applications began with the prepink, but not when the prepink 

 apphcation was omitted. 



Calcium caseinate was thoroughly mixed with sulfur at the rate of 5 pounds of 

 calcium caseinate to 95 pounds of sulfur. This sulfur-calcium caseinate dust was 

 used in three orchards in plots adjoining plots dusted with sulfur only. In every 

 case, apple scab was controlled better by sulfur alone than by sulfur "vvith calcium 

 caseinate added. 



Plots were also dusted according to a schedule which included the use of a copper 

 dust for the prepink and pink, or the pink applications followed by sulfur dust for 

 later apphcations. At two of the orchards, there was less scab following the use 

 of sulfur throughout the season than when copper dust was substituted for sulfur 

 for the applications before the flower buds opened. At the other orchard, there 

 was only a negligible difference between the amounts of scab following the two 

 different methods of treatment. No experimental evidence- was secured to indicate 

 an advantage in using copper dust instead of sulfur dust for the early apphcations. 

 Control of Primary Infection on Leaves. 



As the season advances, the work of the fungicide and the conditions under 

 which it acts become entirely different from what they were at the first of the 

 spraying season. There is naturally and usually an increase in the mean tempera- 

 ture. At the time of the early apphcations, all infection is from the winter spores. 

 As soon as scab lesions appear on the young leaves, there is an increasing possibiUty 

 of infection b}^ the summer spores. In some springs, the trees are for a few days, 

 or even a few weeks, in danger of infection from both the winter and summer spores 

 at the same time. In the absence of sufficient moisture the ejection of the winter 

 spores may be prolonged until after the appearance of scab on the leaves. 



In 1923, ejection of winter spores from the dead leaves beneath the trees was 

 first observed on May 2. Spore ejection continued till June 16, after which none 

 was observed. The first scab symptoms were found on the leaves May 22. It is 

 evident from this that the first winter spores to be ejected did not infect the trees, 

 for the incubation period with these spores of the apple scab fungus was found by 

 Wallace (4) to be eight to fifteen days. During the period in which winter spore 

 ejection continued, there was rain on ten days. Whenever the leaves were wet by 

 rain, winter spore ejection was stimulated, but it was fully as abundant on certain 

 days when the leaves were wet with heavy dew. Winter spores were not ejected 

 in relatively great numbers at any time the last spring. Wallace (loc. cit.) reports 

 that in order for winter spores to infect the trees, the trees must remain wet 8 to 

 10 hours. Such was the condition on May 12 and 21 and on June 8, and it is prob- 

 able that much of the primary infection took place on these dates. 



In order to secure more information on the relative fungicidal efficiency of the 

 several treatments, the percentages of infected leaves on trees June 18 were de- 

 termined. In the Frost sprayed orchard, there were at this time 39 per cent scabby 

 leaves on the check trees, and from 0.1 to 0.3 per cent scabby leaves on the sprayed 

 trees, with onlj^ negligible differences between the several spray treatments. All 

 the spray treatments were practically successful in preventing the primary in- 

 fection. The results were essentially the same in both of the Knights orchards, 

 where there were 15 and 20 per cent scabby leaves on unsprayed trees, and 0.1 to 

 0.2 per cent scabljy leaves on sprayed plots, again with negUgible differences between 

 the different spray treatments. 



Upon examining the dusted orchards, it was found that the primary infection 

 had not been as satisfactorily prevented. In one orchard where the dust treat- 

 ments began with a prepink apphcation, the check trees had 41 per cent scabby 

 leaves, and the dusted trees had 8 to 9 per cent scabby leaves, with only minor 

 differences between the different dust treatments. In another orchard dusted in 



