This section is from the book "The Standard Cyclopedia Of Horticulture Vol2", by L. H. Bailey. See also: Western Garden Book: More than 8,000 Plants - The Right Plants for Your Climate - Tips from Western Garden Experts.
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 remarkable examples of this habit. In the case of the oat-smut fungus, Ustilago avenae, 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-scattered 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 germinate, 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 discovered (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 mycelium 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, Colletolrichum 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 germinating 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 distributed by each succeeding visitor. They soon penetrate into the tender tissues of the blossom, causing the blossom blight. From these blighted blossoms, sucking insects like the aphids carry the bacteria to the tips of the rapidly growing shoots when in sucking sap they introduce the organisms and twig blight follows.
The striped cucumber beetle is probably the chief disseminator of Bacillus tracheiphilus, which causes the cucumber-wilt.