Tree Damage Alert No 86
Whats Wrong With Our Willows?
Willows come in many shapes and forms. Better known species include the white willow (Salix alba), and its varieties such as the Cricket-bat willow (S. alba var. caerulea) and the golden willow (S. alba var. vitellina), the weeping willow (Salix 'Chrysocoma') and the crack willow (S. fragilis) which is a common waterside tree of lowland parts and native to Britain. Unfortunately, several diseases affect willows, and some of the most noticeable appear on shoots and leaves. Symptoms of these various diseases can be very similar, making them difficult to tell apart. For added confusion, more than one disease may often affect the same tree!
Willow scab and Black canker
Two very similar leaf and shoot diseases are Willow scab and Black canker. The symptoms they produce are alike and although the diseases are caused by different pathogens the same species of willow tend to be affected. The earliest symptoms, irregular black spots, can be seen in spring on young leaves and shoots. These enlarge, causing blackened, shrivelled leaves. Discrete black lesions develop on shoots, and if these enlarge the entire shoot becomes blackened and withered. Under the right conditions, infections can be numerous and spread throughout the tree very quickly and the leaves of affected trees look fire blasted.
There are a few minor differences that may help you distinguish between scab and canker. The Black canker pathogen tends to attack leaves and shoots later in the season than scab and can even cause cankers on woody twigs and small branches. Scab, however, only attacks the current year&Mac226;s shoots. Willows most susceptible to scab and canker include Golden willow, Crack willow and twisted or contorted willow (S. babylonica 'Tortuosa').
Marssonina canker and leaf spot
Marssonina canker, otherwise known as anthracnose, is a disease of weeping willow. In spring, brownish spots appear on leaves and shoots of the current year. As parts of the leaves die they become twisted and distorted. Lesions on the shoots also cause distortion and even shoot death. Severely damaged trees look bedraggled and sparse. With repeated attacks, trees may even lose their weeping habit.
The culprits
Scab is caused by the fungus Pollaccia saliciperda, sometimes also known as Venturia. Black canker is also a fungal disease and is caused by Glomerella miyabeana (imperfect stage Colletotrichum). Yet another fungus, Marssonina salicicola, causes the anthracnose of weeping willow.
As with many other leaf and shoot diseases, persistent wet weather during the period when new leaves and shoots emerge results in severe infections. In the case of scab, cool wet weather encourages the most damage while Black canker spreads most rapidly when the weather is warm and wet. The spring weather this year has certainly encouraged the development of Black canker in willows in the south east of England at least, and the disease is clearly visible on trees growing along waterways.
What should you do?
As these diseases are so weather dependent, trees badly damaged in one year may escape infection in another. The onset of dry weather also brings a rapid reduction in new infections. Most trees recover quickly even from severe attacks, so chemical control is rarely warranted in ornamental plantings.
David Rose, Joan Webber & Sandra Denman Forest Research, Alice Holt Lodge, Farnham, Surrey, GU10 4LH
Further information
Rose, D R (1989). Scab and black canker of willow. Arboriculture Research Note 79/89/PATH.
Rose, D R (1989). Marssonina canker and leaf spot (anthracnose) of weeping willow. Arboriculture Research Note 78/89/PATH.
Strouts, R G and Winter T G (2000). Diagnosis of ill-health in trees (2nd Edition). The Stationery Office, Norwich.
This is one in an occasional series of Tree Damage Alerts produced for the benefit of the arboricultural profession and issued by the Arboricultural Advisory and Information Service.
August 2003