Gardening Laidback Gardener Tip of the Day Plant diseases

Juniper and Apple Trees: Not a Good Mix

 

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Cedar-apple rust on apple leaves.

Two of the most commonly cultivated ornamental plants in North America are junipers and apples/crabapples. You see them side by side in gardens everywhere, you see them lined up one next to the other in nurseries… and yet these are two plants you should never be grown close to one another. The recommended distance between the two is 500 feet (150 m): in other words, you should not grow crabapples if your neighbor 5 houses down has junipers, or vice versa!

The disease involved is cedar-apple rust (Gymnosporangium juniperi-virginianae), so  named because it was first discovered growing on “red cedar”, one of the common names of Juniperus virginiana. (Note that it does not infest any of the true cedars (Cedrus) nor any “white cedars” (arborvitae or Thuja), only junipers).

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Gall on juniper.

It’s one of those strange diseases with two alternate hosts. During one phase, it attacks junipers (Juniperus spp.), causing the growth of brown galls that repeatedly produce orange, gelatinous pustules in the spring. During the second phase, the disease must necessarily move, via wind-blown spores, to an apple or crabapple (Malus spp.) where symptoms include yellow spots on top of the leaves and brownish growths underneath. Fruits are also infected. In the very worst cases, it can even cause crabapple or apples to drop all their leaves.

Of course, you could treat this disease every two weeks from spring to late summer with a fungicide (a sulfur-based product would be appropriate) and pruning out the galls on junipers might also help (at least if your neighbors also collaborate), but why even bother? There are rust-resistant apples and crabapples you could try (ask a reputable nursery to point them out), plus not all junipers are subject to the disease either (North American juniper species in general are the most highly susceptible). Or simply avoid the issue by planting junipers and crabapple/apple trees far from each other.

Other Juniper Rusts

There are very similar diseases (other rusts of the genus Gymnosporangium), all of which spend part of their life cycle on junipers, but whose alternate hosts are other trees and shrubs of the Rosaceae family: hawthorns, serviceberries, mountain ashes, pears, quinces. So to make your life easier, never plant them near junipers either!

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

1 comment on “Juniper and Apple Trees: Not a Good Mix

  1. “The recommended distance between the two is 500 feet (150 m): in other words, you should not grow crabapples if your neighbor 5 houses down has junipers, or vice versa!”

    Not possible in Calgary Alberta, where junipers and crab apples are staples. There’s hardly a yard that doesn’t have at least one.

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