80 FUNDAMENTALS OF FRUIT PRODUCTION 



varieties produced in non-irrigated orchards. The reason Kes in the 

 lower atmospheric humidity of the sections where irrigation is practiced 

 and is in no way directly connected with the irrigation. 



This russeting of the skin is often attributed to the action of certain spray 

 materials and without doubt is sometimes partly or even entirely caused by 

 them. In most cases, however, atmospheric humidity is an important contribut- 

 ing factor. The following quotation from a report by Morse, ^^ who has made a 

 study of the subject particularly as it relates to spray injury, is instructive: 



"One of the most prominent facts shown by the tabulated results of 1916 is 

 the relatively high per cent, of russeted fruit on each plot, even on the un- 

 sprayed check which showed 20.57 per cent. This duplicated a condition which 

 prevailed in 1913 when over 31 per cent, of russeted fruit was obtained on the 

 plot upon which no insecticide or fungicide was applied, and the different sprays 

 produced a corresponding increase in amount. Although this russeting was 

 materially increased by different sprays it is evident that much of it must be 

 attributed to natural causes. The weather conditions of 1913 and 1916 were 

 remarkably similar in many ways, and differed from previous seasons in which 

 abnormal fruit russeting did not occur. In 1913 the first spray apphcation was 

 followed by a month of unseasonably, cold weather, with frosts and cold, north- 

 west winds, associated with much cloudiness and heavy rainfall. In 1916, 

 similar conditions prevailed previous to and following the first application. 

 This was also followed in 1916 by heavy rains and continuous cloudy weather 

 in June after the second application, which was not the case in 1913." 



In extreme cases this russeting may be accompanied by cracking and 

 malformation of the fruit, resulting in considerable loss. Sorauer"^ 

 notes that in the grape similar atmospheric conditions may lead to the 

 development of cork pustules on the peduncles or pedicels as well as on 

 the fruit. The cork generally starts to develop under the stomata and 

 the disorder is likely to make its initial appearance comparatively early. 



Some of the effects of high humidity previously mentioned, for 

 example increased leaf surface and the russeting of fruit, are phenomena 

 that likewise accompany a decreased light supply. This raises the ques- 

 tion as to whether a part of the apparent direct influence of atmospheric 

 humidity may not be due in reality to its action in intercepting light. 



Fruit Setting. — Inquiry shows that atmospheric humidity is often of 

 greater importance in the setting of fruit than is generally realized. 

 Hot drying winds at blossoming time may evaporate the moisture from 

 the stigmatic secretions and thus prevent the germination of the pollen. 

 Extreme atmospheric humidity may interfere with the work of insects 

 in carrying pollen or it may encourage the development of certain fungi 

 such as brown rot and apple and pear scab that work on the flowers and 

 destroy or injure them. The well known effects of rain during the blos- 

 soming season in preventing pollination, in washing away and destroying 

 pollen and in diluting stigmatic secretions may be mentioned. A study 



