Chris has planted out several hundred plants of the tender Geranium maderense along the roadside outside of the Gardens. They have survived two winters and by May they should be putting on a spectacular show of flowers. Nothing quite like this will have been seen in this country before so, all being well, it should be spectacular.
However, to achieve this has been a challenge. The principle challenge has been to get them through the past two winters unscathed and it looks like this has been achieved. However, what wasn’t expected was the these normally pest- free plants would be affected by disease.
in early February, Chris noticed that the leaves were becoming infected by a leaf spot fungus causing brown patches of dead tissues on the leaf. As the weeks progressed, these patches spread and lots of the older leaves were killed off, making a sorrry sight.
We began to investigate what was causing the problem, with the help of plant pathologists at RHS Wisley. It turns out that the culprit is a fungus called Coleroa robertianum. This is a common leaf spot on wild Herb Robert leaves late in the winter. There are plants of Herb Robert carrying this fungus throughout the Gardens. What appears to have happened is that the fungus has jumped species to infected the big, juicy leaves of Geranium maderense, which it finds very much to its taste.
This is the first time that RHS were aware of this fungus affecting Geranium maderense and it is interesting that the English name they give to this plant is Giant Herb Robert. Although they are very different in size, the two Geranium species must be quite closely related. Pictured below are the leaf spot infections, and fungal spores of Coleroa robertianum.
We are hoping that, as the weather warms up, our Geranium maderense will start growing rapidly and produce clean young leaves followed by spectacular flower heads. This is what they should look like:
Geranium maderense is an endemic species confined to Madeira and the Canaries