1024 DISEASES AND INSECTS 



DISEASES AND INSECTS 



combat it. The following illustrations will serve to 

 explain and impress this point. 



Plowrightia morbosa, the fungus causing black-knot 

 of plums and cherries, requires two seasons to complete 

 its life-cycle. The first season there appears on the 

 knots only conidia, followed the second season by a 

 crop of ascospores, produced in perithecia, which form 

 a black crust on the surface where the conidia were 

 earlier produced. Other pathogens like Exoascus 

 cerasi, the "witches broom" pathogen of the cherry, 

 lives from year to year as mycelium in the branch and 

 twigs of the broom-like growths it excites, producing 

 each season a crop of spores on the under sides of the 

 leaves. The blister-rust fungus of the white pine, 

 Cronartium ribicolum, also lives from season to season 



1283. The tufted shoots of peach yellows. 



in the tissues of the pine, producing each spring a new 

 crop of spores. This pathogen exhibits another habit 

 peculiar chiefly to certain of the rust fungi, namely 

 that it has another stage or spore form on an entirely 

 different host plant, in this case, the currant, especially 

 the European black currant. The apple-rust pathogen, 

 Gymnosporangium macropus, exhibits the same habit, 

 passing the winter in galls formed on the twigs of the 

 red cedar. In the spring spores appear on these galls, 

 which germinating in situ give rise to other minute 

 spore bodies, the sporidia. These sporidia are carried 

 by the wind to the young apple leaves and fruit, giving 

 rise there to the rust disease so destructive to certain 

 varieties like the Mclntosh and York Imperial. The 

 spores formed on the rusted leaves and fruit of the 

 apple are carried to the cedar, originating a new crop of 

 galls and thus completing the life-cycle. 



While some pathogens may develop in both living 

 and dead tissues of their host, as we have seen in the 



case of the apple-scab fungus, other pathogens like the 

 rust organism just described or the potato-blight 

 pathogen, Phytophthora infestans, require to be con- 

 stantly associated with the living tissues of their host 

 The last-mentioned fungus passes the winter as myce- 

 lium in the tissues of diseased tubers, grows from thence 

 up through the new shoots, slowly killing them and 

 forming thereon the first crop of coaidia, which, carried 

 by the wind to nearby healthy plants, produce the 

 primary infections of the season. The successive crops 

 of conidia produced during the season on the blighted 

 tops are washed into the soil by the rains, find their 

 way to the newly formed tubers, and, infecting them, 

 complete the seasonal cycle of the parasite. 



Many fungous pathogens are now known to pass from 

 one generation of the host plant to the next through 

 the seed. The smut parasites of cereals afford remark- 

 able examples of this habit. In the case of the oat- 

 smut fungus, Ustilago avense, the spores ripen as dusty 

 black masses in the panicles of affected plants just as 

 the healthy plants are in blossom. At this time the 

 two hulls inclosing the grain are open. The wind-scat- 

 tered spores lodge in the open flowers against the young 

 kernel where they are soon safely housed by the closing 

 hulls. They lie dormant along with the ripened seeds 

 until they are planted. Then as the oat kernels ger- 

 minate, the smut spores do likewise, sending forth their 

 germ tubes which penetrate the young oat sprouts 

 before they emerge from the hull. The mycelium grows 

 along up through the growing oat straw, finally giving 

 rise to the black spore masses in the unfolding panicle. 

 In the case of stinking smut of wheat the seasonal life- 

 cycle of the pathogen, Tilletia tritici, is much the same, 

 except that the spores are disseminated at threshing 

 time. Some very important differences in the habits 

 of the loose smut pathogens of wheat, Ustilago tritici 

 and of barley, Ustilago nuda, have recently been dis- 

 covered (1902). The spores of these pathogens are 

 also ripened and disseminated at blossoming time, but 

 on falling within the open blossom they germinate at 

 once, sending their germ-tubes into the tender young 

 kernels. The affected kernels are apparently not 

 injured but continue to develop and ripen. The myce- 

 lium of the pathogen within remains dormant until 

 the seeds are planted and begin to grow, at which time 

 the mycelium also becomes active. It grows out into 

 the young shoots and up through the lengthening 

 culms eventually to give rise to the black spore masses 

 of the smutted heads. The bean anthracnose fungus, 

 Colletotrichum lindemuthianum, is also carried over in 

 the seed. Here the fungus in the black spots or cankers 

 on the pods penetrates into the tender cotyledons of 

 the seed within, goes into a dormant condition as the 

 seed ripens, to become active again when the germina- 

 ting seed lifts these cotyledons from the soil. A new 

 crop of spores is produced, which, if the season be 

 rainy, are splattered on to the stems and leaves of 

 nearby healthy plants and the pathogen becomes 

 established for another season. 



While the wind is the most common disseminating 

 agent of fungus spores, often carrying them for great 

 distances, such agents as rain, flowing water, insects 

 and even man himself, are frequently responsible. It 

 is in the dissemination of bacterial pathogens, however, 

 that insects most generally function. The dreaded 

 fire-blight bacteria are disseminated only by insects or 

 man. They pass the winter in a semi-active state in 

 the half-living tissues along the margins of cankers on 

 limbs or twigs, multiply rapidly with the rise of sap 

 and the heat of spring. They ooze from the affected 

 bark in sticky, milky drops. This ooze is visited by bees 

 and flies, which with besmeared legs and mouthparts 

 fly away to visit the opening apple or pear blossoms. 

 Here they leave some of the bacteria in the nectar 

 where they rapidly multiply, to be more widely dis- 

 tributed by each succeeding visitor. They soon pene- 



