Relation of -the "Weather to Infectiok and to Disease Severity. 



There was rain on nineteen days in May, and ascospores were ejected on each 

 of these days. The combination of rain, viable ascospores, and susceptible host 

 tissue resulted in a condition most favorable to scab infection. In 1924, asco- 

 spores were mature and beginning to be discharged earlier with reference to the 

 condition of apple bud development than in 1923. In Baldwin orchards where 

 no spray was applied before the pink application considerable primary infection 

 was permitted. 



The precipitations for June and July were below the normals for these 

 months. Rain fell on eleven days in June and on seven days in July. As a 

 result of the dry summer, scab control was not difficult, and the year was charac- 

 terised by scab infestation of average rather than exceptional severity. This 

 would seem to indicate that rainy weather before and during the period of 

 flowering may not have any greater influence on the percentage of scabby 

 apples at picking time than has the rainfall of the summer. 



Not more than 7.0 per cent of the fruit on any check plot developed black- 

 rot. The rarity of black-rot and frog-eye leaf spot was presumably the result 

 of the relative dryness of the weather in June and July, for the rainfall was 

 abundant in May and was not below normal in August. 



Scab Infection on Leaves. 



On June 9, after the trees had received prepink, pink and calyx applications, 

 accessible leaves were scored for scab lesions. This was only ten days after 

 the first scab infection of the season was found, and the count therefore gives 

 us a very fair indication of the relative success of the several treatments in 

 preventing the primary infection. The results are given in Table I. 



In all orchards there was somewhat less scab on trees dusted with copper-lime- 

 arsenic dusts at the prepLnk and pink applications than on trees dusted with 

 sulfur. This difference, it should be added, practically disappeared later in the 

 season. 



There was at this time less scab infection on the leaves of sprayed trees than 

 on the leaves of dusted trees, due probably to the more rapid removal of the 

 dust by the frequent rains in May. 



Lime-sulfur solution (with or without spreader), Bordeaux mixture and dry- 

 mix sulfur-lime were practically equally successful in preventing the primary 

 infection. 



Set op Fruit as Affected by Scab and by Fungicides. 



Scab infection on pedicels resulted in the early fall of many flowers and young 

 fruits on check trees at the Harvard Fruit Farm. Flowers were counted on 

 marked limbs in each plot at this orchard, and on June 30 the fruits which had 

 set on these limbs were counted. In sprayed or dusted plots from 3.0 to 5.0 per 

 cent of the flowers had set fruit and on check trees from 0.9 to 1.5 per cent of 

 the flowers had set fruit. The fungicides, by preventing pedicel infection, 

 increased the set of the fruit. 



Control of Scab on the Fruit. 



These results are given in Table II. In considering scab control, it is first 

 of all necessary to take into account the degree of scabbiness of fruit on check 

 trees. The percentages of scabby fruits on check trees in the several orchards 

 were as follows: 



Pine Crest Orchard (check for sprayed plots) 69.4 



^ Pine Crest Orchard (check for dusted plots) 45.8 



Harvard Fruit Farm (check for sprayed plots) 81.0 



Harvard Fruit Farm (check for dusted plots) 84.2 



Middlesex Fruit Farm (check for dusted plots) 50.2 



Results in the sprayed plots. — Four applications of lime-sulfur solution with- 

 out spreader may be regarded as the standard treatment. With this treatment 

 at Pine Crest Orchard there was 1.2 per cent scabby fruit, and at Harvard Fruit 

 Farm, 0.2 per cent scabby fruit. This adds to the evidence, already abundant, 

 that lime-sulfur is a dependable fungicide against apple scab. 



