Pear Trellis Rust & Beech Scale

Issue: 
September-October 2007

IN THE PAST FEW YEARS, there have been an increasing number of calls coming in regarding Gymnosporangium rust-like symptoms on the leaves of ornamental pears. Gymnosporangium is the genus of fungus that is responsible for rust diseases like cedar-apple, cedar-hawthorn and cedar-quince rust. This year, there have been an overwhelming number of reports of rust symptoms on ornamental pears across much of southern Ontario. Samples in early summer were confirmed as Gymnosporangium sp. but the actual species identification required a specific DNA test or aeciospore morphology (aeciospores don’t appear until late summer). Recently, DNA tests have revealed the identification of the fungus to be Gymnosporangium sabinae (a.k.a. G. fuscum, pear trellis rust). The pathogen is pathogenic to species of Pyrus (pear), Juniperus (juniper) and one species of Cupressus (cypress).

The life cycle of Gymnosporangium rusts are complex, often involving obligate, alternate hosts (heteroecious) and various types of spores so bare with me. This rust fungus overwinters on juniper twigs in the form of blunt swellings or protuberances. Sporulation on the juniper host takes place in May and June during warm, humid and rainy weather. These spores travel via wind to the required alternate host: pear. The spores infect the pear leaves and cause orange spots that enlarge to become bright orangey-red and yellow lesions by early summer (Fig. 1). Near the end of summer, unsightly, crème coloured sporulating strands grow out from the undersides of the pear leaves (Fig. 2) and sometimes on fruit. These spores will be carried by wind back to juniper hosts where they will cause new infections on the twigs and overwinter on this host.

Pear trellis rust overwinters on the juniper (Juniperus) host only, not on the pear. It looks as though all species of pear (Pyrus) seem to be susceptible (I couldn’t find any reference to tolerant species or cultivars). Pear trees only become infected if junipers are sporulating within a reasonable proximity to the pear foliage. What is a reasonable proximity you ask? Well, I don’t know. (If I ever recorded a pop song, it would be based on Madonna’s big hit “Sorry” in which I would croon “I don’t know” in several different languages). But if I had to guess, I would suggest a minimum of 100 metres. By removing as many susceptible juniper hosts in the area as possible, you should be able to at least reduce the amount of pear trellis rust on the pear host. Thankfully, not all species of junipers are susceptible to this disease. Juniperus horizontalis, J. communis and J. squamata seem to be resistant to pear trellis rust and can be grown near pears without concerns for this species of rust. However, J. communis is still a host of cedar-hawthorn rust (Gymnosporangium globosum) and cedar-quince rust (G. clavipes) and should still be avoided near pear trees for that reason. But hey, that still leaves us with a lot of options for beautiful, low-growing juniper specimens for areas where ornamental pears are the vertical focal points. Where junipers are the desirable species, avoid planting pear trees within 100 to 1000 m of the junipers. Fungicides may also help protect the susceptible hosts during the various infection periods.

By the 1970s, this disease was widely distributed in Europe, Asia and North Africa. The geographic distribution of this disease in North America started out quite small. Pear trellis rust was first recorded in Canada (British Columbia) in the 1960s, prior to that it was only known to occur in one county of California. At that time, there were no other known sites of this fungus in North America and so the disease was regulated as a quarantined pest in hopes of eradication and slowing its spread. BC developed a certification program for pear and juniper nursery stock based on clean stock, sanitation and a large separation (1 km) between the two alternate hosts: juniper and pear. During the late ‘90s, pear trellis rust was detected in Washington and from there, the US and Canadian governments considered de-regulating this disease. Since then, the disease has become more common in the lower mainland of British Columbia and now we are seeing the rust rear its ugly head in southern Ontario. Check out the factsheet Pear Trellis Rust in BC. For excellent photos of Gymnosporangium rusts, try searching “Gymnosporangium sabinae” and “photo” or “image.”

Beech Scale and Beech Bark Disease
Beech Scale (Cryptococcus fagisuga) crawlers will be hatching soon and over a long period, from about mid-late August to October. Crawlers are very tiny (<1 mm), brown, flat, mobile insects that search out suitable feeding sites often choosing bark cracks and crevices as their permanent home. Crawlers are very mobile on wind and can spread to healthy hosts in this manner. Adult females are also very tiny (~1 mm) but they are covered in a white, wooly mass when mature making them easier to detect this time of year. Beech scale can be found on the bark of large beech trees (&Mac179;40 cm DBH) of American (Fagus grandifolia) and European (F. sylvatica) beech, mostly on the trunk and on the major limbs. Although the beech scale insect does not kill the tree, it predisposes the tree to other problems and creates wound sites that facilitate the entry of beech bark disease (Nectria coccinea var. faginata and N. galligenea). Beech bark disease is a devastating fungus that has caused the death of several native and introduced beech trees in Ontario and North America. Heavy scale populations cause localized tissue necrosis, leading to bark cracks that facilitate the entry of the pathogen. The pathogen infects the cambium (vascular tissue) and eventually girdles mature trees over 5-10 years. Tiny, orangey-red fruiting bodies on the bark are a sure sign of the Nectria fungus. Oozing wounds, cankers and canopy thinning are also typical for beech bark disease.

So far, the scale and the disease have been found in beech forests throughout much of southern Ontario (including cottage country). Monitor scale populations for crawlers and treat at peak crawler hatch. Use repeated applications of insecticidal soap and horticultural oil throughout the egg hatch period since these products do not persist very long. Frequent treatments with a strong stream of water will help wash off eggs and crawlers. Other registered chemical insecticides can be used during peak egg hatch. To assess effectiveness of insecticidal efforts, use a pin and a hand lens to determine whether fluid can be squeezed out of the crawlers or nymphs. Dead scale insects will be dry and flake off easily. Fall and spring (dormant) applications of horticultural oil may also reduce overwintering beech scale nymph populations. Hopefully by reducing populations of beech scale, we can reduce the entry of the devastating pathogen that seems to follow. Unfortunately, insecticidal efforts are only effective where the fungus has not yet entered the vascular tissue. Avoid moving infested beech firewood between July and November during the crawler stage. Whenever I get a call from a distraught homeowner over the decline of their tree, I can’t help but be a little sad. And although I love many types of trees, the mighty beech is one that always makes me feel a little weak in the knees and well, a little humble too. It’s one of those trees I can’t walk by without touching. Check out the Natural Resources Canada, Forestry Images and the City of Toronto Parks, Forestry and Recreation websites for excellent photos and information.

Fall Tree Health
Yes, it has been a hot, dry summer. It has been particularly difficult for newly planted trees and shrubs but older, more established specimens have also suffered, especially in the urban landscape. It is important to remember that fall is another period of active root growth for many perennial (herbaceous and woody) plants. Adequate soil moisture and nutrients will be required to support this root growth. Even if we do receive some precipitation in late August and September, it will take several precipitation events to replenish soil moisture to a sustainable level. Supplemental irrigation every 10-14 days will still go a long way in supplying root systems with moisture and soluble nutrients. Soil tests should be conducted every other year to assess nutrient status and prescribe appropriate rates of fertilizer components. Despite long-standing myths, root growth and flowering will not benefit from large amounts of additional phosphorus to Ontario soils. Roots can only absorb and use so much. Soil test values above 40 ppm (parts per million) are considered high and plants in those soils will not benefit from additional phosphorus in the fertilizer. Soil test values above 60 ppm are considered excessive, they will not support more root growth or flowering of plants compared to those on soils with 40 ppm phosphorus. In fact, excessive levels of phosphorus can actually lead to deficiencies in the uptake and mobility of other nutrients. Avoid using fertilizers with high phosphorus (e.g. 10-52-10) unless soils are actually phosphorus deficient. Nitrogen and potassium are examples of more significant nutrients that are taken up by woody and herbaceous perennial plants in the fall. Up to 1/3 of the annual nutrient applications can be applied in early autumn to take advantage of the root growth period, but there needs to be some moisture in the top 30-45 cm of soil in order for roots to absorb these nutrients.

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.


P: 519-824-4120 ext. 52671 • F: 519-767-0755
jennifer.llewellyn@omafra.gov.on.ca

OMAF website: http://www.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp

Nursery Landscape Agriphone: 1-888-290-4441

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