One of the diseases that people are going to find a lot of in summer, is powdery mildew. This normally forms a white, dusty coating on the upper surface of leaves where the fungus is growing and producing spores. On most plants, the disease is encouraged by previous drought that makes the plants more likely to be successfully infected. Rain, or morning dew, which sits on the leaves then enables air-blown spores to initiate disease.
Although in the countryside, or in gardens, many plants can become infected with powdery mildew within a very short space of time, this isn’t due to the disease spreading from one species to another. There are hundreds of different powdery mildew fungi and most of them can only infect one species of plant. A few can infect more, but usually the affected plant species are all closely related to one another. The epidemics of powdery mildew disease that we see are usually caused by many different species of fungus who are all taking advantage of perfect conditions for growth.
Oak powdery mildew makes a huge show and is easily spotted. Young plants and suckers are very susceptible. These become covered in white, and leaves that have been infected while still developing will be distorted. But the species of powdery mildew fungus that is causing this hasn’t been with us for long. Erysiphe alphitoides appeared on oaks in Europe in the early years of the 20th century. It spread rapidly across Europe and then started to be found around the world. The fungus appears to have host-jumped from mangos. Because oaks have not co-evolved with this disease-causing fungus, they have little natural resistance.
In 2008 the Royal Horticultural Society reported evidence that this fungus had now host-jumped again to Wisteria. On Wisteria the symptoms are different. Very little white mycelium is seen but brown marks occur on the leaves and the disease spreads rapidly (right).
What is it that has made this particular fungus capable of host jumping across plant families twice in only one hundred years? If plant pathologists can work out why a fungus has this potential, and how the host-jump occurs, there may be lessons that can be transferable to animal systems. This might help us to understand, and protect against, host-jumping disease-causing organisms such as the coronavirus that has caused Covid-19.