Worcestershire Record No. 22 April 2007 p. 34

PEAR RUST

Roger Umpelby

I read with interest the article on Page 49 of Worcester Record Number 21, about the pear leaf gall, Gymnosporangiium sabinae. In July 2003 a neighbour in Ashton-under-Hill asked me to look at one of their pear trees which had orange spots all over the leaves. Although I had studied pests of commercial fruit crops for about 40 years, I had never seen this before and didnít know what it was, but I realised it was a disease. I gave a sample to a colleague locally who is a professional plant pathologist who also had worked with commercial fruit for many years. After consulting another professional plant pathologist in Kent we had the answer, Gymnosporangiium sabinae, common name Ďpear rustí. Evidently this disease is quite prevalent on commercial pears in warmer parts of the continent, but had not been reported on crops in the UK for at least the last 25 years.

Pear tree leaves heavily affected by Pear Leaf Rust. 
Picture © Roger Umpleby

Underside of pear leaf showing warty development of the Pear Leaf Rust gall.
Picture © Roger Umpleby

Then I started looking out for it, and I found it even closer to home, at a very low level on the cultivar Doyenne du Comice in my own garden a few days later! Over the next two years I confirmed it in a number of private gardens in east Worcestershire on both very young tees planted for less than two years, and very old trees. It was also present on some old pear trees in hedgerows between Ashton and Elmley Castle. Then in September 2005 I confirmed it on an unsolicited sample from a commercial perry pear orchard in Herefordshire, so it is definitely increasing.

The first signs of it are on the upper surface of the leaf when small distinct oval-shaped yellow-orange patches appear. These then develop a darker centre as the very unusual fruiting-body forms on the lower surface of the leaf. Although both the symptoms and the life-cycle are quite different to most rust diseases, the effect on the trees and the conditions favouring development of the disease are the same for all rusts. Leaves badly infected will turn yellow and drop but, although young trees may be weakened, the disease does not kill its host.

Rusts do not normally have a second/alternate host, but because spores can carry for many miles, it is not necessary to have the secondary host near to pears for the disease to successfully complete its life-cycle.

Although rusts may appear to worst in dry weather, it is purely that they release their spores in dry weather, in fact the disease spreads very rapidly and infects most readily in warm wet conditions. The wet late summer and autumn last year favoured rust infection and it is already very common on grasses (including lawns), and a number of other garden and wild plants. Given the mild wet March this year, and the recent deluges, I expect pear rust to be very common and severe again this year.

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