Tag Archive | Diseases

Begonia hemsleyana: SPECIES TALK – MARCH MEETING

Begonia hemsleyana, Bot. Mag. 125: t. 7685 (1899).
Source: Curtis Botanical Magazine; Author: Hooker

Carmel Browne presented the Species Talk on 16 March 2013.

The area of distribution of this species extends from northern Burma to the Chinese province of Yunnan in moist, upland forests.

B. hemsleyana was introduced to Kew Gardens by way of seed collected in south Yunnan in 1899. It was named in honour of William Hemsley who worked on Chinese plants at Kew at that time.

B. hemsleyana is rhizomatous, jointed at or below the soil with erect stems. The leaf blade is palmately compound, glossy green, sparsely hairy between the veins, paler green beneath with a reddish tinge. The petioles are pink with short, woolly hairs. Flowers are pink and fragrant.

I chose to speak on this species today because this is only the second time it has flowered for me. It has been described as difficult. From my experiences, I have found it requires a cool, moist, well lit situation with good air movement. Because it naturally produces short, closely spaced stems, good air circulation is essential to keep fungal diseases at bay. A well-drained premium mix that is allowed to dry between waterings suits B. hemsleyana.

B. hemsleyana, B. rex, B. pedatifida and B. circumlobata are closely related and all belong to section Platycentrum. B. hemsleyana has been successfully crossed with Rex Cultorum begonias. B. ‘Raspberry Swirl’, B. ‘Picasso’ and B. ‘Hula Skirt’ are the results of such crossings. I do not know if these have ever been grown in Australia.

Understanding Mildews

Powdery mildew, species Podosphaera fusca

Powdery mildew, species Podosphaera fusca

Fungus and mildews are always a problem during humid weather. Understanding how they grow can help when you are trying to control them. Fungi are often visible to the naked eye and are often named for their appearance.Powdery mildew is a group of related fungi, usually showing as whitish spots on leaves or new shoots. They live on the surface and send hollow tubes into the plant to suck out nutrients. Some powdery mildews attack a range of plants – some only attack one plant, or at the most, two or three. Powdery mildew is worse in humid weather, and once it has got a hold, it will keep growing, even in dry weather.

Downy mildew is also a group of related fungi, also worse in humid weather. The infected patches appear first UNDER the leaves. Downy mildew grows within a plant and sends out branches through the victim’s stomata (the microscopic openings in the leaves) to create pale patches on the leaves. The problem usually disappears in dry weather, or sometimes if you improve air circulation or stop overhead watering.

Spacing plants to allow good air circulation will help to control some fungal diseases. Environmental problems such as heavy rain, very hot sunshine, strong winds and consistently high night temperatures can also lead to the development of these diseases. The soil should be kept rich in soil organisms to provide conditions that are favourable for the vigorous growth of beneficial fungi and bacteria that will feed on other more destructive types.