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Figure
1. Bacterial leaf spot of English Ivy. Note light coloroed transluscent
halo around lesion.
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The English ivy (Hedera helix) is a hardy evergreen vine
that is grown widely as a ground cover under trees and in other
areas where it is difficult to grow grass. English ivy is a decorative
plant on walls, rocks, and other rough surfaces, or trained to cover
a trellis. Window boxes and hanging baskets are also attractive
settings for ivy. English ivy grows best in shade and on the surface
of buildings where the walls are in at least partial shade. The
plants have woody stems and climb by aerial rootlets that cling
readily to brick or masonry and less so to wood.
In Illinois, the English ivy is commonly attacked by one or more
leaf spot diseases. In some cases, stem cankers are produced by
the same organisms that cause leaf spots. The more common leaf spot
diseases and their controls are described in this report.
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BACTERIAL LEAF SPOT AND STEM CANKER
Bacterial leaf spot and stem canker, caused by Xanthomonas campestris
pv. Hederae (X. Hederae), is the most common disease of
English ivy. It is first seen as small, circular, dark green, water-soaked
(oily) lesions on the leaves. As the disease develops, the spots enlarge
into roughly circular to angular areas with greenish brown, water-soaked
margins and reddish brown to black centers. The margins of lesions on
older leaves are reddish or reddish purple, but the water-soaked appearance
is evident on the underside of the leaf, especially when it is held up
to the light. Frequently, a yellow (chlorotic) halo surrounds older lesions
(Figure 1). With alternating wet and dry periods, the centers eventually
become dry and cracked.
Under warm, wet conditions, extensive black cankers develop on the stems
and petioles. Infections may cause the portion of the plant that extends
beyond the infected area to crack, girdle, and wither along the length.
When cankers develop on the stem, the plant fails to grow normally and
remains dwarfed and unthrifty, with yellowish green foliage. Stem infections
may occur directly through the susceptible new growth. A soft, dark brown-to-black
decay occurs rapidly in the stem tissues and advances until the older
tissue is reached. Twig tips turn black and die back into the old wood.
Bacterial leaf spot and stem canker is most common on leaves that are
shaded and close to the ground, especially near water spigots and other
wet areas. The bacterium enters the vine through natural openings, such
as stomates or hydathodes, and wounds and is spread principally by splashing
water and persons working among wet plants. The organism survives between
growing seasons in the soil and in plant refuse.
FUNGAL LEAF SPOTS
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All fungal leaf spot diseases of ivy, although caused by different
organisms, have one major factor in common: they require a film
or droplet of water on the leaf surface for the spores to germinate,
grow, and invade the plant tissue. Thus, the most important control
measure for these diseases is to keep the foliage as dry as possible.
In greenhouses, avoid overhead watering; water only to the potting
media and roots. Proper sanitation is important to eliminate these
causal fungi from growing areas.
These fungi all produce spores that are disseminated by splashing
water (rain or irrigation) and by persons working among infected
plants when they are wet. The spores of certain fungi are also windborne
or transmitted from one plant to another by sticking to the bodies
of insects and mites. The spores germinate in water and invade leaves
through natural openings as well as directly. These fungi overseason
in diseased leaf and stem tissues.
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Figure
2. Colletotrichum leaf spot. Left, small and large spotson leaf;
right, close-up of leaf lesion with dark specs (acervuli and setae)
in center (photo, Dept. of Plant Industry, Florida Department of
Ag. and Consumer Services).
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1. Colletotrichum leaf spot, caused by the fungus Colletotrichum
trichellum, appears as irregular, generally dry, brown to reddish
brown spots up to about 3/4 inch (2 cm) in diameter (Figure 2, left).
Many of the spots spread inward from the margin of the leaf. Black specks
(fungus fruiting bodies or acervuli), containing black, needlelike structures
(setae), commonly appear within the lesions on the upper leaf surface
(Figure 2, right). Lesions on the petioles and stems may result in premature
defoliation, dieback of the stems, or even death of plants in scattered
patches.
2. Scab or Sphaceloma leaf spot, caused by Sphaceloma
hederae, appears as small, raised, round to irregular spots with reddish
brown margins and grayish white, slightly depressed centers that are later
sprinkled with dark fungus fruiting bodies (sporodochia). The spots are
often numerous and may merge to form irregular blotches.
3. Minor fungal leaf spots reported on English ivy include those caused
by Phyllosticta concentricta, P. hedericola, Cercospora hederae,
species of Coniothyrium and Corynespora, Sphaeropsis hedericola
(S. hederae), and Ramularia hedericola. These uncommon to rare
diseases appear as small to large, round to irregular spots of various
colors, often with conspicuous margins or concentric rings. Infected leaves
may occasionally wither and fall prematurely, giving the plants a ragged
appearance.
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