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Excessive pruning should be avoided because it greatly reduces the crop for that year . By this we mean that , within a given pixel , the position of radar scatterers has not changed to any significant degree , but the ensemble of scatterers ( i.e ., the entire ground surface within that pixel , as well as some adjacent ones ) has moved up , down , or sideways in some correlated fashion . Slice the stem lengthwise to help you diagnose this disease . When purchasing an apple or pear tree , you should know both the scion and the rootstock you ’ re getting . Grafting is useful however , for more than reproduction of an original cultivar . If you plant a pollen-sterile variety , you may need to plant a third variety in order to get fruit on all three pear trees . Varieties and Rootstocks : Apples Selecting Apple and Pear Varieties Apple flowers must receive pollen from another variety of apple to produce fruit . ‘ Liberty , ’ ‘ Redfree , ’ and ‘ William ’ s Pride ’ are all scab resistant . Control : Cut and destroy all infected blossoms as soon as they droop or die . For now , however , sanitation ( promptly removing and destroying fallen fruit and leaves ) , cultural practices designed to promote optimum tree health , and perhaps occasional physical or chemical intervention are all that is normally needed to obtain a satisfying crop in most years . Sclerotia produced by white mold are brown-black , resemble rodent droppings , and are usually found on the inner stem of infected plants . Annual drawings help when you're making garden plans during the winter , and are a good way to keep an ongoing record of what you purchased each year . Spray all plant parts to thoroughly wet foliage and soil . During the process of repotting , prune decayed roots and provide proper soil drainage . This disease can cause young leaves to curl and turn purple . Most dwarfing rootstocks grafted to pears in colder regions carry an OH X F designation followed by a number . At that point , caterpillars have grown to about 1 to 2 inches in length . Sclerotia are the overwintering structures for this fungus and they can survive for many years in the soil . In principle , all three compo-nents of surface displacement could be obtained from space ; in practice , we are only likely to obtain at most two , if data from both the ascending and descending passes of the satellite are available . The quality of the fruit from these disease-resistant trees is acceptable , although it may not measure up to that of some other varieties recommended in Table 1 for zones 4a and 4b . The most common above-ground symptom of crown and root rot is a rapid yellowing of foliage and dropping of flower buds . The fungus may also infect stub ends of stems from which flowers have been cut . Adequate plant spacing , maintaining the relative humidity below 85 % and providing good air circulation provide excellent control for this disease .