The Exobasidiales are, as their name suggests, an order of basidial fungi and Exobasidium is a major genus within the group. Other sub-groups/parallel groups occur. All these fungi possess a basidium but they do not produce a fruitbody/sporocarp as such and this distinguishes them from the true basidiomycetes. Possession of a basidium does not necessarily mean the fungus is a basidiomycete. Instead these fungi produce a hymenium on the surface of the host tissue and this is where the basidia and basidiospores are formed. All are obligate parasites on flowering plants, especially Ericaceae.
All the species show a marked degree of host specificity and this feature together with spore shape, size and degree of septation allows for adequate discrimination. Spores may be crescent-shaped, banana-shaped, or cylindrical with or without cross-walls. They are useful in characterising species, but clearly defined macroscopic differences usually render this unnecessary. The type of damage to the host plant is a useful identification character. Exobasidium produces symptoms ranging from coloured leaf spots with no thickening of tissue, through typical leaf blisters and apple-galls to deformed shoots with mis-shapen and discoloured leaves. The spore producing surface is usually found on the underside of affected leaves but may cover the whole shoot. In some species infection is localised to individual leaves and must be continued annually from dispersed spores. In others the mycelium is clearly systematic and perennial, moving from one stem to another via the rootstock. In other species whole branches are attacked, but not via the rootstock. When established in the host plant the fungus will not kill it but may make it more susceptible to frost damage and this may be the first obvious evidence of infection. some hosts are affected by more than one species of Exobasidium but the symptoms are usually distinctive and the microscopical characters of the fungus are a reliable guide to identity.
Both altitude and latitude are important for distribution patterns, but other factors exist. For example, there are fewer high level species found in Britain than in the boreal regions of Scandinavia due to the loss of natural coniferous forests above 1000ft/2000ft in our northern areas. In this country the majority of species are to be found on low-altitude plants. In the boreal regions of both Scandinavia and Switzerland species vary on the same host plant according to altitude.
The following species were illustrated by slides:
E. rhododendri This fungus produces distinctive red apple-galls on Rhododendron species only. While it is very common and conspicuous on wild plants in the alps it is not common on plants in gardens. The slide was taken at Kindrogan, Perthshire and the specimen was the first to be recorded in Britain. It has been widely reported in this country but only reliably so in five catchment areas of Scotland. The fungus affects the flowering of the plant.
E. horvathianum This rarer species has appeared on Rhododendron ponticum in the Black Sea area and was reported as having appeared in England on R. luteum in 1922 but no specimens or data are available. It produces large fleshy galls up to 7cms in diameter on the leaves.
E. japonicum Known as the Azalea gall. This species is very common wherever the hosts, the evergreen Asiatic azaleas imported from Japan, occur. The gall is first greenish yellow, changing to orange and red and the modified leaf is swollen to look like an apple. Though common and widespread in the British Isles, it was 25 years before it was recorded in Flintshire.
E. camelliae Produces conspicuous swollen pinkish fleshy swellings on the various parts of Camellia japonica. It is fairly common in the south of England and spreading. It may attack the teaplant, Camellia sinensis, when it constitutes a major disease which can be controlled only by burning.
Other rarer species, worth looking out for, are as follows:
E. unedonis This is a rare Mediterranean species which affects the leaves of the Strawberry Tree (Arbutus unedo). The young shoots are clustered and open earlier than healthy ones forming a conspicuous tassel. The leaves are small,yellow and then red. This fungus also appears very rarely on plants in temperate gardens. There is a single U.K. record from N.E. England.
E. karstenii Frequently appears on shoots of Andromeda polifolia (Bog Rosemary) which bear much enlarged leaves. These redden and become purplish-black before falling off. The spores are curved, up to 18um long and septate. The host plant is not common and likely to be affected whenever found – which probably accounts for its rarity. It has been recorded locally at Flaxmere, Delamere.
Parasites which affect the "berry plants":
E. vaccinii Is a common and conspicuous parasite forming red leaf pouches on pink leaves of Vaccinium vitis-idaea (Cowberry). However it rarely affects more than a single leaf on a shoot. It is common on moorland and in high pine forest and has been recorded from 13 sites in England, 4 in Wales, 39 in Scotland. It is clearly under-recorded, although it may take considerable searching to find it. Empetrum nigrum (Crowberry) is affected by this fungus only.
E. juelianum Attacks the whole Cowberry plant which becomes dwarfed, heavily branched and covered with pale pink hymenium. It is rarer than the previous species and found on only a few mountain sites in Scotland and North Wales.
E. oxycocci and E. rostrupii Both species occur on Vaccinium oxycoccus (Cranberry). The first is decidedly rare in Britain, in spite of the widespread occurrence of its host. It is conspicuous so not likely to be overlooked when stands of Cranberry are searched. The second species forms inconspicuous red spots on otherwise unaffected leaves and the underside of the leaves carries an inconspicuous pink hymenium. It is common and widespread and probably occurs wherever the host plant is found, but because it is so inconspicuous, it is greatly under-recorded.
E. myrtilli Is widespread and common causing swelling, enlarging, and reddening of the leaves of shoots of Vaccinium myrtillus (Bilberry). It is the commonest species of the genus in the British Isles, in keeping with the spread of its host.
E. expansum Attacks Vaccinium uliginosum (Bog Bilberry) an uncommon plant in Britain. A few plants are known on Helvellyn. The only British record of the fungus is from the Cairngorm area.
E. sydowianum and E. uvae-ursi Both attack Arctostaphylos uva-ursi (Bearberry). The first is very rare, known only in Scotland from Cairngorm and three sites in Wester Ross. It forms conspicuous yellow, thickened waxy spots on the leaves of the plant. However, the host plant is frequent in Scotland and the second fungus which produces enlarged, mahogany coloured shoots and is not yet known in the U.K.is also worth searching for.
NOTE: This account was compiled from notes made at the A.G.M. by Rita Cook and John Taylor and two papers produced by Bruce. These are:-
1. "Exobasidium in the British Isles" - Bruce Ing - "The Mycologist", Volume 12, Part 2, May 1998.
2. "Exobasidium in Scotland" - Bruce Ing - Botanical Journal of Scotland, 51(2) 221 – 225.
At present there is no handbook.
John also quotes the following sources:-
Nordic Macromycetes, vol.3; a few in Ellis& Ellis – Microfungi on Land Plants; some illustrations in Fungi of Switzerland, vol.2.
