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 110°F.
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.