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.

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