|Discula destructiva Family: Gnomoniaceae
Common name: dogwood anthracnose
Host plant: Cornus florida (flowering dogwood); Cornus nuttallii (Pacific dogwood)
|Description & Life Cycle: This anthracnose fungus attacks the lower leaves of its preferred host tree and works its way up the tree to the uppermost leaves in a severe case of infection. Dogwood anthracnose is identifiable by its tan blotches surrounded by a purple rim. (The purple rim may not appear for a few days.) In some cases, the centre of the blotch area becomes so thin holes will appear within the purple rim.
Infected leaves on Pacific and flowering dogwoods usually drop prematurely in the spring. However, those leaves at the branch tips which may become infected in the fall will stay on the tree and cause the death of terminal buds. The result is a reduction of bud break in spring which forces the tree to produce new leaves through lateral buds in midsummer.
Flower buds and fruits are also susceptible to infection.
In the fall, any infected leaves typically stay on the branch allowing the anthracnose fungus to travel along the leaf petioles into the branch tips which results in oval-shaped cankers developing on the branches. These grow until the cankers completely girdle the branch, effectively killing it. If the fungus spreads to larger branches, these can also succumb to the infection…and if it reaches the main trunk, the whole tree can die.
This is a nasty fungus and, sadly, the cool, wet spring and fall weather we experience in the Pacific Northwest are exactly what Discula destructiva thrives on.
Special Notes: It is thought the dogwood anthracnose came from somewhere in Asia, arriving first in the Port of Seattle in 1976 and then in New York in 1978. From these two ports of entry, the fungus has spread into Washington, Oregon, Idaho, Connecticut, Massachusetts, New Jersey, Delaware, Pennsylvania, West Virginia, Virginia, Maryland, North Carolina, South Carolina, Tennessee, and Georgia. Two Canadian provinces, British Columbia and Ontario, also have the dogwood anthracnose fungus.
To give some scale to the seriousness of this fungus, a survey of Cornus florida trees was conducted in Catoctin Mountain National Park in Maryland in 1984, which determined only three percent of dogwoods were not infected with Discula destructiva. Four years later, a follow-up survey revealed eighty-nine percent of the dogwood trees had died, there were only a very, very few dogwood seedlings regenerating new growth in the forest, and all remaining live trees were infected.
Remedial Action: none known
Posted on March 22, 2018
|Peronospora spp. Family: Peronosporaceae
Plant affected: Aquilegia (columbine)
Thankfully not found in our garden.
Description: Affected columbine plants will show yellow patches on the upper side of their leaves. These patches are distinguishable from natural leaf variegation by their straight lines. They also tend to initially run alongside the main leaf vein before the disease spreads to the rest of the leaf. You may see a whitish-purple fungal growth on the underside of the leaves on affected plants. Eventually, with the progression of the disease, leaves will curl and turn brown before falling off.
Flowers are not immune either. Affected blossoms will look as if they have been soaked by rain and become distorted. You may see purple spots developing on flower stalks. Eventually, flowers drop off. If the plant becomes infected after flowering and as seed pods are forming, you will see brown spots on the pods. These will not set viable seeds.
Aquilegia downy mildew (ADM) is spread by air- and water-borne spores. Can be carried long distances by the wind. Outbreaks typically occur when there are prolonged periods of rain in spring and/or late summer. Ideal conditions for production of spores and infection.
While the airborne spores are only viable for a short time, the fungus is capable of producing a second type, called a resting spore, between the leaf tissues. These resting spores are more resilient. Released into the soil from decaying diseased material they can survive for several months, and perhaps for several years although this fact has not been confirmed.
Special Notes: This virulent disease was first recorded in UK gardens in 2013. However, the Food and Environment Agency of Great Britain was aware of Aquilegia downy mildew prior to this date. It appears to be restricted solely to the UK, at this time.
The disease is so new, scientists have yet to assign a species name to the genus. While it has been placed in the same genus (Peronospora) as powdery mildew which also affects aquilegias, it is not related…despite certain similarities in symptoms.
Remedial Action: There is no chemical control for this disease. However, strict attention to hygiene in the prompt removal of any diseased parts or whole plants, as a non-chemical control method, will reduce the risk of resting spores contaminating the soil. Immediately burn diseased plants, or bag them for garbage removal. DO NOT COMPOST.
The Royal Horticultural Society (RHS) has recommended not re-planting aquilegias in an affected area of the garden for at least a year. This advice may not have merit as there other species of Peronospora affecting other plant species which produce resting spores capable of surviving several years in the soil.
Read more about Aquilegia Downy Mildew (ADM) in my Blog dated March 1, 2017. (Quick link here.)
Posted on February 14, 2018
|Helleborus net necrosis Family: Carlavirus
Common name: hellebore black death
|Description: A viral disease, Helleborus net necrosis virus or HeNNV for short, which infects plants in early to mid-spring. Symptoms appear as distortions on new foliage, causing the plant to look stunted. Typically, patterns of vein-clearing (streaks of lighter leaf colour along the veins) will become evident, followed by discoloration and darkening into distinct black streaks, or lines. The damage will become progressively worse as spring advances. Black streaks will eventually develop on the stems and extend onto the sepals and carpels.
Special Notes: Hellebore Black Death has been known to develop a pattern of rings, rather than streaks, which can confuse a gardener into thinking their plant is afflicted with the much less deadly disease, black spot.
Early speculation leans towards this virus being carried and transmitted by the hellebore aphid, Macrosiphum hellebori.
Remedial Action: Remove all leaves which are showing black streaking or black spots and place in garbage. Do not compost! Check plants carefully for aphids, as this is how scientists believe the virus is being transmitted. Treat thoroughly if aphids are present.
If the plant is deteriorating with progressing black streaking and overall stunted growth, dig up plant and place in garbage. There is no none cure for Hellebore Black Death at this time.
Posted on January 24, 2018
Gymnosporangium sabinae Family: Pucciniaceae
Syn. Gymnosporangium fuscum
Common name: pear trellis rust; European pear rust; pear-juniper rust
Host plants: Juniperus spp. (juniper) – primary; Pyrus spp. (pear) – secondary
Life cycle: Spring – spores are wind-blown from junipers onto pear leaves
Description & Life cycle: This is a rust fungus which requires two host plants from unrelated groups – juniper and pear. In spring, wet weather triggers the production of orange jelly-like masses of spores from a spindle-shaped gall on the juniper. (These galls are typically found on the tips of the tree branches.) The wind carries these spores for distances of 4 – 6 miles (6.5 – 10 km) until they come into contact with a pear tree species. Symptoms on the pear tree first appear as orange-yellow spots on young leaves. Small branches and young fruit can also be infected. The leaf spots brighten to reddish-orange over the summer and will develop tiny black dots in the centre. These are pycnia, or fruiting structures. They play a role in the development of the spore structures which are formed later in the summer on the underside of the leaf, directly under the colour spot. In late summer, brown blister-like swellings are forming, looking quite like miniature acorns. These are the aecia*. Spores (called aeciospores) are produced in this structure and are released into the wind through the fall months to land on juniper hosts where the spores infect young branches. Fungus overwinters on the juniper host.
*Note: The aecia of pear trellis rust can produce spores for two years in a row. Aecia of apple-cedar rust only produce spores for one year.
Both plant hosts, juniper and pear, are needed for this rust fungus to complete its life cycle. However, the fungus will sometimes overwinter on pear trees in branch galls and produce new pear infections the following spring.
Special Notes: Originally native to Europe, and totally confined to this region, this rust fungus was identified in North America in the early 1930s, likely imported on ornamental junipers. The spread of this fungus was positively identified in Victoria, BC in 1961 and throughout other regions in the Pacific Northwest, including parts California, in the 1990s. Ontario reported cases in 2007; Michigan in 2009; southeastern New York State in 2011; Connecticut in 2012; Virginia in November 2014. It is also making inroads in the United Kingdom.
This fungus can cause losses in pear crops, although this appears to be truer in Canada than in Europe.
Remedial Action: Control is mainly concentrated in separating the two host plants by as much distance as possible. Recommended is minimum 100 ft (30.5 m); preferred 1000 ft (305 m). Unfortunately, these separation distances may be inadequate given the spores are carried on the wind and can travel quite a distance…up to 6 miles (10 km). If possible, prune any galls spotted on juniper species before April 1st. But, the galls can be hard to spot on juniper trees…especially when they are not producing their masses of spores. (When reproduction gets going in spring, the spindle-shaped gall produces bright orange gelatinous spores. Much easier to spot on a dark branch.) Prune galls which develop on infected pear trees and remove any infected pear leaves, fruit and small branches by the middle of August.
There are a few rust-resistant juniper species available now. These include: Juniperus horizontalis (creeping juniper), J. squamata (Himalayan juniper), and J. communis (common juniper)
Posted on October 29, 2016