Home > Begonian > Volume 72 (January/February 2005, pages 8 - 11)

Powdery Mildew in Begonia
by Antoon Hoefnagel

Introduction:

Powdery mildew (Oidium begoniae) is next to botrytis one of the most important fungus diseases in pot plants.

Plants infected with powdery mildew are easily recognizable through the presence of a white layer of fungus threads, usually on the upper surface of the leaves or flowers.

This layer is easily removed with one's fingers because powdery mildew only grows superficially.

Severe damage to the plant is easily detectable, because it may also result in diminished growth.

 In the U.S. problems with powdery mildew also arise in Poinsettias.

Powdery mildew also leads to problems in Gerberas and cut roses.

In different plant families not all species have the same sensitivity, sometimes there are varieties that have a level of resistance to powdery mildew.

There are also differences in resistance to powdery mildew within the Begonia family.

If this resistance could be included next to the magnificent leaf colours and forms and the beauty of the flowers, begonias should give much more pleasure to all of us.

The fungus:

Powdery mildew is caused by the fungus Oidium begoniae and belongs to the section Ascomycetes. Oidium begoniae creates generative and vegetative spores. On the basis of these spores we are able to determinate the family of the fungus. The fungus is called oidium if the spores are vegetative. In general powdery mildew can only infect a specific plant species. This means for example that the powdery mildew on a Saintpaulia does not infect a Begonia.

On the other hand it means that the powdery mildew of a Begonia cannot infect a Saintpaulia. Fungus families, that cause powdery mildew are obligatory parasites . The syndrome of mildew can vary by species and by cultivar. The reaction of the plant to the fungus has influence on this syndrome. Microsphaera begoniae creates smooth white spots on one Begonia cultivar, while it creates an irregular shape of mildew on another one. The powdery mildew created by M. begoniae on Begonias infects leaves, but infection of the flowers is also possible.

A mildew colony begins with a few traces on the leaf surface. Out of these traces grow hyphal threads which branch off several times. Special cells (haustories) grow out of these hyphal threads through the leaf surface and settle in a surface cell of the leaf (epidermis cell). The fungus receives nutrition from this epidermis cell of the plant. These hyphal threads continue to grow and multiply regularly. A colony can be visible within days. This depends on which mildew species is involved. This is also called the latency period. From the hyphal threads grow new spores, on these spores new spores are produced. Under the microscope this is visible as a chain of spores.

Left, above, powdery mildew on leaf and below Cleisothecium. At right, above Condiospores and below, Hypal threads.

When the spores are mature they can easily come loose and can be spread by the wind. When the spores come down on a susceptible leaf they can germinate and form a new colony. This cycle can be completed within days if the circumstances are favourable. This cycle is the vegetative cycle of the fungus. There is also generative cycle , but this is rarely involved in the development of a disease in horticulture.

Incease and Distribution:

Infection by mildew is highly dependent on temperature. Most powdery mildews have an optimum temperature of 21°C (Celio & Hausbeck, 1998). The germination of spores is less efficient at higher temperatures. Also the growth of hyphal threads will be curbed at higher temperatures. There is no infection if the temperature is above 30 ,(C. Research has found that the maximum temperature is 30 °C for appresoriumformation, while the colonies die at a temperature of 32°C (Quinn & Powell, 1982). Mildew will not die at lower temperatures, but its growth will decrease. Although mildew is not dependent on liquid, humidity can be important for the infection. The fungal spores contain 70% liquid, so they do not need water from the outside to germinate. (Agrios, 1997). So infection is possible at low humidity, in contrast with other fungi like botrytis and rust, these need high humidity for germination and infection. Never the less some research shows that high humidity promotes infection of mildew.

Condio sporesat epidermis.

Furthermore humidity influences spore formation and the release of spores. Sudden changes in humidity can result in release of the spores. Finally there is also literature describing infection in combination with light (Quinn & Powell, 1982). Light can affect the release of spores, because maturation is dependent on light. In contrast, light can have an effect on the lifespan of the spores. Increased radiation results in a lower survival rate of the spores. The effect of climate on epidemic mildew is complex; so it is difficult to give an unambiguous advice to change it so that you can control infection. It must be recommended to prevent big fluctuations in the climatic circumstances.

Control:

Between cultivars within one plant species there can be differences in sensibility to mildew. By hybridisation the resistance can be crossed into new cultivars. Through on the years various publications have been issued about the sensibility to mildew in plants. (De Gelder & vd Wurff, 1994; Strider, 1976; Strider, 1980). Resistance breeding is still insufficiently used for improvement in the florist sector.

In practice the mildew in potting plants is controlled by the use of chemical fungicides, or such as sulphur vaporisers.

Next to chemicals, it is possible to take growing measurements to avoid the spreading of infection. You could think of removing the dead plants or plant parts, avoiding draft spots in the greenhouse and realizing a climate with no big fluctuations.

Research shows that silicium provokes thicker cell walls, especially in the epidermis of the leaf. With this treatment plants will resist mildew infection better. There are several examples with 'plant enhancers'; at first sight they give good results. The only disadvantage is that the plant have to be sprayed at regular intervals; and therefore the plants will be more wet and that is not to recommended.