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STRANGB FACTS ABOUT FRUIT DISEASES 



Apple Scab. 1, This disease is not caused by a single organism, 

 but by a group of closely related strains or varieties of the species, 

 Venturia inequalis. The Mcintosh strain is only feebly parasitic on Baldv;in 

 and Yfealthy, while the Yfealthy strain is not injurious to Mcintosh, but 

 causes severe damage to Wealthy foliage and fruits in some of the mid-westem 

 states. 



2. In order for the scab fungus to complete its life cycle, a typical 

 sexual process is necessary. While primary infections are taking place in the 

 spring, half of the ascospores coming from the old leaves are male and half 

 are female in character. All of the summer spores, including their progeny 

 throughout the period of secondary infections, resulting from a male asco- 

 spore are likewise male in character. The corresponding situation holds true 

 for the progeny of the female ascospores. Late in the season when a leaf in- 

 fected by both male and female forms of the fungus drops to the ground and 

 dies, the scab fungus threads which formerly were confined to the region 

 betv/een the cuticle and epidermis, grow into the interior of the dead leaf 

 where some of the cells of the male threads unite with cells of the female 

 threads, thus effecting fertilization. This union is the beginning of the 

 perithecium that bears the dreaded ascospores which start the scab trouble 

 anew for the grower in the spring, 



3. It is possible to cross different strains of the apple scab fun- 

 gus. This has been done by initiating leaf infections from single ascospores 

 of opposite sex from different scab strains and then harvesting the perithecia 

 the following spring that develop in the infected leaf. The outcome is nevi 

 strains of the scab fungus that behave differently from the original parents 

 in degree of parasitism on the varieties concerned. Similar crossings occur 

 very commonly in nature with many kinds of plant disease fungi, giving rise 



to new physiologic strains of the standard species and varieties. In fact, 

 it constitutes a terrible handicap to plant breeders and plant pathologists 

 who attempt to develop varieties of plants resistant to certain diseases. 



Fire Blight. For many years it was supposed that the only vmys in 

 Vifhich the bacteria which ooze out from holdover canlcers may spread from one 

 place to another was by the splashing action of rain and by the flight of 

 insects which feed upon or walk through the milky drops of bacterial ooze, 

 A few years ago, it was discovered that the same sticky drops of ooze may be 

 and commonly are drawn out into fine threads many feet in length and blown 

 to distant points. This process provides another means of dissemination of 

 the fire blight organism. 



Peach Leaf Curl , Th© seasonal behavior of the fungus causing this 

 disease is quite different from that for most other fruit disease organisms. 

 In fact, it is not thoroughly understood yet in spite of much research on 

 the subject. No one has ever been able to find evidences of twig and bark 

 infections where the fungus could exist in either active or dormant condition 

 during the long period of summer, fall and v.-inter when the disease is con- 

 sidered to be in an inactive stage. In only a very fev/ instances have there 

 come reports that the curl fungus is able to attack the peach fruit. Other- 



