Woody Plants BI237 Week 11

FOREST PATHOLOGY -- CANKERS

While vascular wilts disrupt the xylem, canker-causing fungi kill phloem or inner bark.

A canker may appear as a sunken area that the stem has continued to grow around.

Cankers are usually caused by Ascomycete fungi. Cankers can also be caused by frost or sun-scald, by rust fungi, or by stem-rot basidiomycetes.

We don't know much about how the fungi enter the tree. They are not supposed to be able to penetrate intact bark. Presumably they use wounds.

EXAMPLE 1: CHESTNUT BLIGHT

American chestnut (Castanea dentata) was the dominant tree in the eastern US. It was beautiful, made valuable timber, and produced high-quality food. Adult trees were eradicated in a matter of decades following the introduction of the fungus in from Asia to NY in 1904.

The ascomycete that causes chestnut blight is Cryphonectria parasitica. The "sacs" containing the ascospores have narrow necks that protrude through the bark. Ascospores are wind-dispersed. There are also conidia (asexual spores) produced. These are sticky rather than dry and can be carried by insects or birds. It is believed that puncture wounds caused by insects are the most common method of entry. The fungus feeds upon the inner bark and cambium.

Lots of dedicated people are working to revive the American and European chestnuts.

1. We are now in the third generation of a back-crossing program to introduce resistance genes from the Chinese chestnut into the American chestnut.


2. There is also research on a hypovirulent strain of the fungus. Hypovirulence literally means "lesser virulence." Hypovirulent isolates have a piece of double-stranded RNA, which doesn't normally occur in fungi. It is now considered that the dsRNA is a virus, and essentially causes a disease in the fungus, making it less virulent. So far, the hypovirulent strains can be used as an inoculant but appear to be unable to spread naturally, due to vegetative incompatibility.

CANKER EXAMPLE 2: SUDDEN OAK DEATH

Sudden oak death (SOD) is a new disease that became established around the San Francisco Bay of California in the mid ‘90’s and has since been found in southern Oregon. It shows every indication that it may develop into one of the most devastating diseases to hit North American forests.

Hosts:

Tanoak (Lithocarpus densiflorus) is highly susceptible and trees of all sizes are rapidly killed. Oaks in the red oak group, coast live oak (Quercus agrifolia) and black oak (Q. kelloggii) are commonly infected on the main stem and large branches and killed from such cankers. California bay laurel, rhododendron, and other understory species show foliar lesions or stem cankers. In addition, many other species that occur outside the current distribution of the disease, such as northern red oak, are susceptible in seedling inoculations, leading to concern for how much devastation may eventually result.

The pathogen:

The pathogen is a newly described species, Phytophthora ramorum. It is in the same genus as the fungus that caused the 19th century potato blight in Ireland. It produces at least two kinds of spores: some that can be dispersed with plant matter or soil; and others that germinate directly and are probably wind-dispersed.

Origin of the disease:

The fungus is almost certainly introduced to California, and the same fungus has since been found in Europe, but the origin and nature of the introduction are unknown. Virtually all the facts point to a recent introduction, perhaps from an ornamental Rhododendron:

1.The disease was unknown until a few years ago.

2.The disease is devastating, with little resistance in many of the native hosts.

3.The disease appears to be rapidly expanding its distribution in North America.

4.Genetic variation of the pathogen in North America is very limited.

5.The pathogen has been found in Europe in Rhododendron nurseries.

Epidemiology of Sudden Oak Death

Strangely, sporulation has not been detected on stem and branch cankers and must be rare if it occurs at all. Thus, the lethal disease on woody hosts apparently does not contribute to inoculum. Abundant sporulation has been found on foliar lesions in the field and can readily be induced on lesions in the lab. Lesions on leaves dried for several months can still sporulate.Hosts that get lethal cankers probably get infected from inoculum produced on foliar hosts. Experimental inoculations indicate that infection can occur without wounding, both on leaves and stems.

Management

People are trying to monitor the spread of the disease. It is not yet known if climate will restrict its impact to the coast, or if it just hasn't had time to spread eastward yet.

CANKER EXAMPLE 3: A NATIVE!

Hypoxylon canker is a common disease of Populus tremuloides. It is the most important aspen disease in many areas. Stands with up to 70% mortality have been reported. Understocked stands have increased incidence.


Hypoxylon canker on aspen.

CANKER EXAMPLE 4. BEECH BARK DISEASE

Nectria fungi are transmitted by a scale insect. The insect was introduced into Nova Scotia about 1890. We don't know whether this species of Nectria was already here or not. There are other Nectria that seem to be in balance with native tree species. The scale insect makes lots of holes in the bark, where Nectria spores can enter. Sometimes the fungus completely girdles the tree, killing it. In other cases, the tree is able to create a "wall" of undifferentiated tissue called callus that prevents the fungus from expanding. But damaged tissue can provide an entryway for other diseases and insects, which may kill the tree. Some trees appear to be resistant to the scale.

Injured trees send up thickets of sprouts from roots, changing the composition and structure of eastern forests.