Disease problems in the Genus Platanus
Mr John S. Brereton
In Melbourne as in other parts of the world various forms of Platanus are widely grown suiting the parks, boulevards and avenues to be found in urban areas where large ornamental trees (25-30m height, 20-25m width) are preferred. The Plane often tolerates polluted air and compacted soil conditions making an ideal hardy choice for roadsides and streetscapes. The Plane generally exhibits a degree of drought tolerance under Australian conditions but is sometimes a problematic tree selection due to outbreaks of Microsphaera alni, Powdery Mildew and Apiognomonia veneta (asexual:Discula platani) Plane Anthracnose. The latter disease problem is more serious and apart from the obvious aesthetic problems post infection the disease also places the trees under stress. Stressed trees are likely to be more susceptible to other infectious and non-infectious problems including insect attack.
Planes are generally known world wide as being susceptible to Plane Anthracnose a fungal disease caused by Apiognomonia veneta. This disease problem is also refered to in the United States as sycamore anthracnose. The Sycamore (Platanus racemosa) specifically the endemic American form, is generally considered to be the most susceptible to anthracnose. The London plane (Platanus x acerfolia) is less susceptible and Oriental Plane (Platanus orientalis) exhibits a high degree of resistance by comparison. Cultivars of Platanus x acerfolia ‘Liberty’ & ‘Columbia’ are found to be faster growing and more resistant to anthracnose than the parent.
Anthracnose symptoms expressed include the following:
Spring twig blight destroys previous years growth at the tips;
Destruction of buds;
Blight of new growth and young leaves;
Cankers may form in older more mature trees growing at the base of twigs and girdling new shoots;
Major leaf blight causing leaf distortion and marked brown necrotic areas on leaves crossing over the veins. These areas enlarge and will often cover most of the leaf surface. Leaf drop may follow in severe cases. This disease in leaves should not be confused with scorching from heat and drought stress. The distinctive elongated brown lesions on leaves crossing over veins should avoid any confusion with drought stress.
In young trees under nursery growing conditions it may also be more difficult to maintain strong apical dominance as terminal buds are destroyed.
Planes receiving adequate water and nutrients usually re-foliate in summer.
Climatic conditions favouring outbreaks of anthracnose. Outbreaks of anthracnose are more likely when during periods of wet weather in spring and early summer, with the disease surviving and over-wintering
The fungal organism over winters in cankers in twigs and branches
During the Spring spores which lie within infected tissue are produced and released to be spread by wind and rain or water splash.
Management of anthracnose.
- The maintenance of tree vigour is important with adequate nutrient and water supply.
- Removal of infected twigs and branches by pruning is beneficial. This will be usually confined to peripheral growth resulting in limited control of the disease problem
- Under nursery conditions do not use overhead irrigation methods that will result in long periods of leaf wetness and high humidity levels
- Maintain adequate air circulation between trees.
- Raking up infected leaves twigs which have fallen may have little impact on disease control as parts of the tree are already infected and a source of infective spores.
- Applications of fungicides such as copper formulations, triadimefon, chlorothalonil and mancozeb have assisted to contain but not eradicate this disease problem. Fungal applications are usually applied at approximately 10 to 14 day intervals from bud swell. The application must be strictly in accordance with all label instructions. The application method must ensure adequate coverage particularly under nursery conditions.