| Sphaeropsis
blight, previously known as Diplodia tipblight or dieback, is
a major worldwide disease of pines, particularly of 2- and 3-needle
pines in the United States from the East Coast to the Great
Plains and the West Coast. It is mainly a disfiguring disease
of trees growing under stress, with trees showing numerous brown,
dead branch tips (Figure 1). Except for pine and white spruce
(P. glauca) seedlings, Sphaeropsis blight rarely kills the host
because only current-season needles, shoots (or candles), and
seed cones are infected and killed. Where new candles have been
killed several years in succession, however, entire branches
may dieback from "resinous bleeding cankers." |
Click
on image for larger version
Figure 1. Sphaeropsis Blight on an Austrian Pine (Dr.
P.C. Pecknold, Purdue University) |
No
species of pine is immune, but many are somewhat resistant. Austrian
or black pine (Pinus nigra) is the most susceptible followed
by Scots (P. sylvestris), and red (P. resinosa), Swiss
mountain (P. Mugo), ponderosa (P. ponderosa), and
eastern white (P. strobus) in decreasing order of susceptibility.
Douglas fir (Pseudotsuga menziesii), big-cone spruce (Pseudotsuga
macrocarpa), Norway spruce (Picea abies), Colorado blue
spruce (P. pun gens), Sitka spruce (P. sitchensis),
noble fir (Abies procera), silver fir (A. alba), and
American larch (Larix laricina) are occasionally infected.
Other evergreen genera reported as being susceptible include true
cedars (Cupressus spp), Chamaecyparis spp, arborvitae
(Thuja spp), hoop pine (Araucaria cunninghamii), junipers
(Juniperus spp), and Cedrus spp. The Sphaeropsis
fungus should be considered likely to colonize any pine and many
other conifers growing under unfavorable conditions or among diseased,
highly susceptible species such as Austrian pine.
The
disease is caused by the fungus Sphaeropsis sapinea (synonyms
S. ellisii and Diplodia pinea) which normally infects
the young, healthy, unwounded needles of new candles. However, twigs
of trees weakened by stress, such as drought, water shortage, compacted
soil, root injury, excess shade, heat reflected from nearby roofs
and walls, and wounds caused by hail, pruning tools, or insects
(pine spittlebug, Aphrophora parallela, and a bark beetle,
Myelophilus piniperda) are also infected. Trees 30 years
or older are most severely damaged. The disease rarely occurs on
young trees, possibly because the fungus builds up mainly in the
tissues of old, dead seed cones. Thus, until a tree has acquired
a quantity of old of old dead cones, its chances for infection remain
low. Seedlings and young trees can become infected, however, if
they grow near older infected trees. Seedlings are usually killed
when the fungus girdles the stem at or near the soil line. Infected
germinating pine seeds are killed by a decay of the radicle as it
emerges.
Symptoms
The most noticeable symptom of Sphaeropsis blight is a browning,
stunting, and twisting of the new shoots and needle growth (the
candles) (Figure 1). Usually the lower branches of the tree are
the first and most seriously affected, and sometimes infection is
concentrated over a small area or on one side of a tree. In wet
springs, branches over an entire tree may show brown tips. Annual
destruction of many buds and shoots causes gradual decline of ornamental
trees. A brown discoloration first appears near the base of the
needles and then progresses toward their tips. Infected needles
usually die when they reach one-half to three-fourths their normal
length. Generally, all needles developing on an infected shoot die.
Young
candles that grow from infected buds are generally short, bent,
and soon die, whereas, candles already developed from healthy buds
stop elongating and become curled and twisted after infection. As
infected stem tissue dies back, the whole candle droops. Resinous
cankers often appear on infected stems at the youngest branch whorls
and often on stems at the base of blighted needles. Death of the
candle stimulates the formation of new lateral shoots in the healthy
stem tissue below. These shoots eventually may become blighted,
turning shades of yellow-green to straw color.
|
Click
on image for larger version
Figure 2. Austrian Pine Needle Showing Sphaeropsis
Blight (Dr. P. C. Pecknold, Purdue University)
|
Infected
twigs commonly exude resin, resulting in the stunted, dead,
brown needles sticking to the twig. Infected cones fail to
mature. Black, pinpoint-sized specks fungus fruiting bodies
or pycnidia form near the base of infected dead needles, mainly
under the sheath (Figure 2), in the bark of infected twigs
or branches, and on infected second-year cone scales. In older
twigs that have been killed, the bark is dark brown and the
sapwood is stained gray or blue-black. Top killing and death
of entire trees result from a massive invasion of the wood
and bark of trunks and, rarely, the major roots of trees stressed
by an extended drought or other agents.
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Sphaeropsis
blight can be confused with symptoms caused by (1) low temperatures,
(2) drought, (3) winter drying, and (4) Nantucket pine shoot moth
(Rhyacionia frustrana) injury. These problems can be distinguished
from Sphaeropsis blight because the dead tissues beginning in autumn
lack the small black pycnidia. Twigs infected by the pine shoot
moth are also distinguished from Sphaeropsis blight by the presence
of frass and the hollow piths that are eaten out by the moth larvae;
shoots killed by Sphaeropsis blight, in comparison, are solid, hard,
brittle, and infiltrated with resin.
Disease
Cycle
The Sphaeropsis fungus overseasons as minute black pycnidia
in the bark of woody stems, the needles (Figure 2), fascicle sheaths,
seed cone scales, and the outer bark scales on an infected tree
or on the ground. In wet weather, from early spring into late autumn,
microscopic mostly one-celled dark brown spores (conidia) are liberated
from the pycnidia and disseminated by wind and splashing rain, and
in the fluid secreted by the spittlebug (Aphrophora parallela).
The fungus can also be disseminated as mycelium on pine seeds.
The
conidia are capable of germinating within a few hours at temperatures
between 53°F and 96°F (12°C to 36°C). A wet period
of 12 hours is sufficient for spore germination and infection. In
warm weather, symptoms appear 3 to 4 days after infection takes
place. The fungus makes its most rapid growth at about 82°F
(28°C).
| Germinating
conidia (Figure 3) invade new buds or new needle growth by stomatal
penetration and sometimes through wounds. Penetration of young
stems occurs through the intact epidermis. Most infections occur
in mid to late spring. Young shoots are highly susceptible during
the short period from bud swell to when the needles are about
half grown. Second-year seed cones are also highly susceptible,
but their susceptible period appears to start later than that
of the shoots and extends later into the season. The seed cones
of previously noninfected trees are more likely to become infected
than the new shoots. |
Click
on image for larger version
Figure 3. Sphaeropsis sapinea, the Cause of
Sphaeropsis Blight or Diplodia Tipblight as Seen under a lab
Microscope (drawing by L. Gray)
|
The
Sphaeropsis fungus grows quickly through the needle tissue, then
to the stem, and finally into adjacent needles. Dieback symptoms
appear several weeks or more after infection. Later in the season,
pycnidia (Figure 3c) appear on the tissue soon after it dies. Neither
the mature needles or stems of previous years' growth are infected
by the fungus, nor are the first-year seed cones. Pollen cones are
seldom infected.
Control
- Fungicide protection is required from bud swell to when the
needles are half grown. Three sprays are needed, spaced 10 to
14 days apart, starting just before budbreak. Effective fungicides
are listed in the Illinois Urban Pest Control Manual. This manual
is revised annually and should be available in every Cooperative
Extension office. During rainy springs, a fourth application,
10 days after the third, will be necessary. Because new infections
usually occur on the lower branches of a tree, good coverage of
these branches is essential for optimal protection.
- Keep pine trees in good vigor by watering thoroughly during
dry periods, fertilizing regularly according to a soil test report,
and applying insecticides suggested by University of Illinois
Extension Entomologists. Pines stressed by adverse environmental
conditions are, in general, much more susceptible to stem infections.
- Where feasible, remove all infected twigs, branches, and seed
cones from trees, or from the ground in dry weather, when first
noticed. Pruning tools should be disinfected between cuts by dipping
or swabbing in 70% denatured or rubbing alcohol or in fresh household
bleach (1 part bleach in 9 parts of water). The plant debris should
be collected and burned, buried, or placed in a compost pile and
covered with a thin layer of soil.
- Pruning should be done in autumn when the tree is dry.
For
further information on diseases of ornamentals contact Nancy R.
Pataky, Extension Specialist of Turf and Ornamentals, and Director
of the Plant Disease Clinic, Department of Crop Sciences, University
of Illinois at Urbana.
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