Monday, 30 March 2009

Another parasitised spurge – The Cypress Spurge. Euphorbia cyparissias

Uromyces pisi-sativi
(infected plant on far left).

The Cypress spurge is common throughout France, but only occurs here and there in England and Scotland. It is a small plant with many crowded leaves which makes it look a little like a tiny cypress tree. Like the Wood Spurge it is also parasitised by a rust fungus. One photo here shows the normal flowers and the other shows a stem which is elongated and with yellow leaves. All these latter leaves are covered in pustules as in the infected wood spurge and the smell is richly perfumed though rather more sickly than that of the infected Wood Spurge.
There is a difference in the detailed biology. Rust fungi can have very complicated life cycles. That of the rust on Wood Spurge is simplified. It only has one host plant and the life cyucle therefore only occurs on that species. The spores are carried from one host to another by flies.
The rust on the Cypress spurge has two different host species and as in the life of many other rust fungi the two hosts can be very different. In this case the other host is frequently the garden pea. Other species related to the pea can also be infected. The fungus lives in the spurge during the winter and then flies or even bees carry the spores to some leguminous plants (like peas) in the summer. The life cycle continues on those plants carrying through a form of sexual recombination of chromosomes. Then a new form and generation of spores are carried to infect again the cypress spurge plants.

Wood Spurge - the Gardenia of the woods? (*Euphorbia amygdaloides*)

The Euphorbias, alias the spurges, include some of the most statuesque of garden plants. And in the natural woodlands the wood spurge commonly strikes a dramatic pose. Its large clusters of yellowish flowers formed not of petals but yellowish bracts are eye catching. Each plant stands up to eighty cen-timetres high. Each red stem has a perennial drapery of drooping large thick, deep-green leaves which tend to be purple on the underside. The young shoots with their small, still growing leaves usually flop to one side. A thick white latex oozes out from any broken stem. This latex is extremely acrid in taste and can burn the skin. It is hardly surprising that one does not usually see any damage caused by predators. The plant is common throughout France, but in Britain it becomes rare north of a line from the Mersey to the Wash.
Here and there in most springs I see stands of the plants with the topmost drooping growth replaced with clumps of broader pale yellow/green leaves standing straight. The photo compares the abnormal on the left (plus one fly) with the usual form on the right. You might imagine that the yellow form was just an aberration, a variation, but then you notice that groups of flies are settling on them. I have also seen a small yellow slug eating a yellow leaf. Unfortunately when I bring the camera close enough to get a picture the flies tend to fly off.
But pick a deformed stem and immediately there is a powerful and delightful scent. In contrast the ordinary dull green and purple leaves smell musty and earthy. The odour of the deformed leaves re-minds me of gardenias. It is unexpected. One must suppose that the flies and slug are attracted to the smell and are devouring the exudations from the leaves which produce it. But what causes the smell? I remember the experience from long ago of a similarly strong scent emanating from a fungus disease ( a ‘rust’ disease) on creeping thistle. Indeed, examination of the leaves under the microscope show that each yellow leaf bears hundreds of minute red fungal pustules. These each exude an aromatic liq-uid. This it is a ‘rust’ known as Endophyllum euphorbiae-sylvaticae, rarely found in Britain. It only lives on the wood spurge and no other species of spurge. It has contrived to change the shoots of the spurge into yellow scented ‘flower’ forms to attract the flies which would most surely carry the spores of the fungus to new plants. I also wonder if any other animals would eat these soft, yellow, scented leaves?

Monday, 2 March 2009

Bi-monthly Weather report, Gourdon, Departement du Lot, France, January 2009

To view statistics for January & February 2009 (click)
Both months were fairly mild. As usual December was colder with 5 cms of snow on Dec. 26th.
As is so often the case in early spring the days were bright with early ground frosts and the occasional day with quite warm afternoons
First flowerings.
Snowdrop Jan 16th.
Daffodil Feb. 25th.
Celandine Feb. 26th.