Biology Reference
In-Depth Information
widely diverse
types
and may persist
as
known as blue stem. The symptoms appear late in
the season, leaves turning pale, cane tips bending
downward, canes taking on a bluish color, lower
leaves wilting and drying. Death is often delayed
until the season after first infection. Black rasp-
berries are more susceptible than red. The disease
is sometime serious on strawberries, especially in
California, but cannot always be separated from
root rots. Plants may collapse in large areas at the
beginning of hot weather.
Verticillium wilt is very destructive to mint in
Michigan and Indiana, also reported, though not so
serious in Oregon andWashington. Infected plants
are stunted, defoliated, and killed; yield of oil is
greatly reduced. The fungus attacks all species of
mint, but peppermint is most susceptible. There
are some resistant hybrids. Deep plowing,
inverting the soil, has reduced the amount of wilt.
Verticillium
is especially damaging to toma-
toes in Utah and California. First symptoms are
yellowing of older leaves and wilting of tips
during the day; later, margins of all leaves curl
upward, then leaves drop (see
Fig. 3
). Plants are
stunted; fruit is small. Moderately resistant vari-
eties Riverside and Essar have been developed
for California. Symptoms on potatoes are rather
indefinite, but often there is yellowing of lower
leaves, shortening of internodes, and rosetting of
the top (see
Fig. 3
). Resistant varieties may be
symptomless hosts. Verticillium wilt is common
on eggplant and okra, rather rare on pepper. It
occurs on Chinese yard-long bean, rhubarb and
New Zealand spinach.
On herbaceous perennials in eastern gardens
I find Verticillium wilt common on aconite and
chrysanthemum, with leaves turning dark brown
and hanging down along the stem. When the stem
is cut across near the base, a circle of black dots
indicates the fungus in the vessels. Such plants
seldom die immediately but flower poorly and
gradually peter out. Wilt was serious on green-
house chrysanthemums until a wholesale com-
mercial concern started to provide healthy
propagating stock from cultured cuttings. Other
ornamental hosts include abutilon, aralia, bar-
berry, begonia, China aster, carnation, dahlia,
fremontia, geranium, marguerite, peony, poppy,
snapdragon (see
Fig. 4
), stock and viburnum.
a saprophyte in the soil 15 years or more.
Of the ornamental tree hosts silver maples are
most susceptible, then sugar and red maples,
elms, with occasional reports on ailanthus,
alfalfa, aspen, ash, boxelder, beech, black locust,
camphor-tree, carob, catalpa, Chinaberry,
cucumber, deerbrush, dogwood, goldenrain,
horse-chestnut, India hawthorn, redbud, linden,
magnolia, oak, osage-orange, olive, pistachio,
persimmon, periwinkle, Russian olive, sassafras,
strawberry, smoke-tree, tulip-tree, walnut,
mango, sunflower and hickory. Maples may wilt
suddenly in midsummer, often a large branch or
one side of the tree drying and dying while the
other side stays fresh. The sapwood of the
infected side has greenish streaks, and sometimes
slime flux develops on the bark. The disease can
be chronic, progressing slowly for several sea-
sons, or acute, affecting the entire tree in a few
weeks. In elms the leaves may be smaller than
normal, with a drooping flaccidity in hot hours of
the day. Later there is a slight yellowing, deep-
ening until the foliage is a striking lemon yellow.
Defoliation starts at time of first yellowing, and
quite often branchlets drop as well as leaves.
Sapwood discoloration is brown, and the disease
cannot be told positively from Dutch elm disease
without laboratory cultures. Tyloses and gums
are formed in the wood as with other toxin-
producing fungi. The fungus always progresses
upward through the xylem vessels so there is little
danger of downward infection of the main trunk
from pruning operations. Progress is slowed by
adequate moisture and by high nitrogen fertil-
izers, ammonium sulfate being particularly
helpful.
Verticillium wilt is also a problem on rose
understock. Ragged Robin, Odorata, and
Multiflora are very susceptible, Dr. Huey less
susceptible, and Manetti resistant.
In fruit trees the wilt is often known as black
heart or verticillosis. It is common in apricots,
less so in almonds and peaches; branches may
drop their leaves and die. Also susceptible are
sweet and sour cherry, avocado, plum and prune.
On bush fruits - raspberry, blackberry, dew-
berry and youngberry - the disease is commonly
