Types of Garden Enemies
STEM AND BULB NEMATODE (Ditylenchus dipsaci).
As a bulb pest this nematode causes concentric dark rings in hyacinth, tulip, narcissus, and other bulbs, lengthwise dark streaks in iris. Hyacinth foliage may be twisted and split with yellow flecks; narcissus leaves sometimes have pustules or blisters. The stem nematode strain affects phlox, sweet william, and a few other ornamentals, producing narrow, crinkled, or brittle leaves on stems swollen or bent sidewise.

Control. Remove and burn diseased phlox. Bulbs are treated commercially with hot water, 3 hours at 110F.

POWDERY MILDEWS
Some people use the term mildew to denote any blight or mold, but it should be limited to a special type of fungus disease in which the fungus itself is seen as a growth on the surface of plants. Downy mildew is a wet-weather affair with the fungus producing spores from the underside of a leaf; powdery mildew flourishes in periods of high humidity but not when it is actually raining. Powdery mildew is a white felty growth on the surface of leaves, stems, and buds. It is made up of a tangle of fungus threads, mycelium, with spores growing out in chains to give the powdery effect (Figure 42B). These spores, conidia, break off and are carried by wind to other plants. They do not germinate in free water but take advantage of the humidity that comes when cold nights change to warm days or when plants are crowded, or in the shade, or near walls or hedges where there is little air circulation. The fungus enters plant cells only by means of haustoria, little suckers sent in from the surface mycelium, but this action results in a good deal of stunting and dwarfing. Powdery mildew fungi are obligate parasites; they cannot live on dead tissue and prefer vigorous and succulent new growth. Plants over-stimulated by nitrogenous fertilizer are subject to mildew.

In most species perithecia, sexual fruiting bodies, allow winter survival on stems or in old leaves; sometimes mycelium winters in buds. The perithecia are fastened to the white mycelium by means of long appendages which are coiled or variously branched at the tips. The different genera are distinguished by the type of appendage and the number of asci in the perithecia.

CRAPEMYRTLE MILDEW (Erysiphe lagerstroemeriae),
the most serious disease of this host from Maryland to Florida and Texas. The fungus winters as mycelium in dormant buds and in spring covers developing buds with a dense white coating of spores which are rapidly disseminated to all young shoots. Affected parts are coated heavily with a white mealy growth, young leaves are both stunted and abnormally thickened, inter- nodes are shortened, buds often fail to develop, diseased leaves and buds drop. But when hot weather comes along, mildewed tissue may be replaced by nearly normal growth.




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