Blowin' in the wind

Gymnosporangium sabinae on pear leaf

Juniper Hill from the garden

Host leaf cells with fungal hyphae

Aecium with aeciospores

At the end of September yellow spots appeared on the leaves of  the pear tree. No pears, just spots. On the underside  of the leaf, below each spot, was a small wart with little clusters of wispy hairs sticking out of it - top pic. A search through my books, I have a general interest in fungi, and on the web, pointed to Gymnosporangium sabinae, a rust fungus belonging to the family Pucciniaceae. These are a wonderfully diverse group of plant pathogens of simple structure but with complex life cycles typically involving two host plants and four different types of spore to complete. This enables them to spread rapidly when the going is good and to ensure a great potential for variability so as to keep one step ahead of any defences that the host plants may evolve against them. 

Often such organisms are difficult to identify down to species level but what clinched it in this case was the specificity for the host plants involved. G. sabinae has for its hosts the pear tree, Pyrus sp., and the juniper. 

Juniper is not a common plant, but it likes the limestone and it so happens that across the Painswick valley from us is a hill called Juniper Hill, ringed in the second pic. down. At one time there were many junipers on it, now down to just two, and it may be that for centuries past basidiospores have been blowing across on the east wind to infect pear trees and aeciospores have been blowing back on the prevailing westerly to reinfect the junipers. 

Stroud is traditionally a cider producing area, including perry, and the upper slopes of the hills round the town were a stronghold of this production. All the farms round us still have remnants of old orchards.

One of the leaves has been fixed (a treatment like pickling to preserve it from change and decay), embedded in paraffin wax, sectioned to 15 microns thickness, put on a microscope slide and stained. 

The low power shot is of a slide stained with methylen blue and picric acid. It shows the spores contained in the aecium on the underside of the leaf. The aecium has a lining of fungal tissue but most of the tumor-like swelling is made of cells from the pear leaf. They are filled with a latex like substance produced as a response to the infection. The high power shot is of a section stained with safranin and cotton blue. It shows the fungal hyphae growing around the cells of the spongy mesophyll in the leaf. 

If you click on the pics. for a full screen view the magnifications are about X150 and X3500 respectively.