Biology Reference
In-Depth Information
sea-kale, Spanish mustard, wild radish and
Bras-
sica robertiana
. Small, steel-gray to black areas
appear around point of emergence of secondary
roots. Enlarging roots are constricted and turn
black. Rotation is essential for control. Choose
globe rather than long varieties.
Armillaria
Basidiomycetes, Agaricales
One of the mushrooms, cap-shaped on a stalk
with an annulus or ring butno volva (cup) at the
base; gills attached to the stem; spores white
(see
Fig. 1
).
Armillaria mellea
Mushroom Root Rot
of trees
and shrubs, also known as Armillaria root rot or
toadstool disease, first described in America in
1887, known in Europe a hundred years earlier.
The fungus is called honey mushroom, honey
agaric, oak fungus and shoestring fungus.
Although the honey-colored toadstools are often
seen in the East around rotting tree stumps and
may occasionally cause death to weak ornamen-
tal trees, the chief damage is west of the Rocky
Mountains, especially in California, where most
fruit and nut crops and ornamental trees and
shrubs are menaced.
The decay is of the roots and root crown.
Sheets of tough, fan-shaped mycelium are found
between bark and wood, the latter changing to
light tan, becoming soft and watery in texture.
Clumps of toadstools are often found at the base
of dead or dying trees, especially in autumn, but
do not always appear in dry seasons. They are
honey-colored or light tan, with a stalk 4 to 6
inches or more high and a cap 2 to 4 inches
across, often dotted with brown scales. Basidio-
spores formed along the gills are wind-borne.
They can establish themselves in old stumps and
dead trees but cannot infect healthy trees. The
latter are infected in the ground by means of
black or brown cordlike rhizomorphs, the “shoe-
strings,” which grow out from infected roots
a short distance through the soil. On meeting
and penetrating a healthy root, the fungus pro-
gresses along the cambium layer, working up to
Fig. 1
Mushroom Root-Rot Fungus,
Armillaria mellea
and girdling the root crown. Leaves are dwarfed,
turn yellow or fall prematurely; on small trees all
foliage may die simultaneously. On conifers, par-
ticularly pines, there is an abnormal flow of resin
from the root collar.
Trees subnormal in vigor and suffering from
drought are most injured. Orchards of citrus
and other fruits on lands recently cleared of
oaks are liable to enormous damage unless
resistant rootstocks are used. The rot is found
lessoftenondryhillsidesthaninvalleysnear
streambeds, where flood waters deposit soil
andinfecteddebrisaroundrootcrowns,orin
places kept too wet by artificial watering.
Ornamental trees and shrubs are often injured
when extra soil is added in grading and terrac-
ing, and are then kept too wet by watering the
lawn frequently.
The list of susceptible plants is far too long to
be given in entirety. A representative selection
includes almond, apple, apricot, avocado, cherry,
citrus, currant, grape, incense cedar, peach, pear,
plum and raspberry, hickory, filbert and walnut,
California pepper-tree; oaks, pines, spruce and
sycamore; azalea, rhododendron, boxwood and
rose; (root, crown, and stem rot on) African
daisy and (stem rot on) sunflower; and sometimes
other herbaceous plants such as begonia, carna-
tion, dahlia, narcissus, peony,
rhubarb, and
strawberry.
