Apple proliferation phytoplasma
English names: Apple proliferation, Whiches' broom
Nordic names: Æbleheksekost (DK), Heksekost (NO), Häxkvast (SE)
Major host plants
Apples are the main host. Pear trees with symptoms of proliferation have been found, but there has been given no proof of the presence of Phytoplasma mali. Minor hosts are Catharanthus roseus (Madagascar periwinkle), Corylus avellana (filbert), Pyrus communis (European pear) and Vitis vinifera (grapevine).
- On trees: Affected trees lack vigour, shoots are thin and the bark, which is sometimes fluted lengthwise, has a reddish-brown colour. Necrotic areas appear on the bark and some branches may wither. Diseased trees may die. However, in mild infections, they may recover after the shock symptoms of the first 2-3 years and, subsequently, produce normal fruits again, especially if adequately fertilized.
- On buds: The first noticeable symptom is often late growth of terminal buds in the autumn; an apical rosette of leaves develops instead of a dormant bud. However, a more reliable symptom is the premature development of axillary buds during the first 2 or 3 years following infection. They give rise to secondary shoots (witches’ brooms) near the apex of the main shoot in summer. The angle between the secondary shoots and the main shoot is markedly narrow on infected trees. Witches' brooms generally appear successively on various parts of the tree, or all at once over the whole tree, rather than repeatedly on the same branch.
- On leaves: Leaves appear earlier than normal and are finely and irregularly serrated and smaller. Chlorosis, reddening and premature defoliation often occurs. It should be noted that chlorosis and reddening of leaves may be caused by agents other than proliferation; therefore, diagnosis should not be based on this symptom alone. Stipules are abnormally enlarged, and occasionally there may be up to four in number per leaf. Petioles are rather short.
- On flowers: Flowering is delayed, sometimes until late summer or autumn. Most of the blossoms of infected trees are normal, though flower stalks may be abnormally long and petals irregular in shape, unusually numerous or leaf-like.
- On fruit: Smaller, poor flavoured, fewer, paler and broader relative to their length than usual. The stalks are longer and thinner than normal. Seeds and seed cavities are smaller.
Proliferation has only been reported from the EPPO region. However, there are unconfirmed reports from India and South Africa.
A map can be downloaded from EPPO’s website. See instructions here.
After inoculation with an infected bud, symptoms appear in the following year. From infected rootstocks, symptoms appear on the first growth of the scion. Once within the tree, the phytoplasma is mainly localized in suckers and terminal shoots, and it is restricted to, and spreads within, the phloem vessels. The distribution of the phytoplasma in the trees varies during the year. In winter, it declines in the tree but concentrates in the root. During spring, the stem can be reinvaded as the phytoplasma spreads up from the roots, reaching a peak in late summer or early autumn. Very high temperatures may cause the amount of phytoplasma in the aerial parts of the tree to decline or even disappear, in which case symptoms may not develop.
The possibility of introduction with infected plant material is relatively high because of symptomless infections. Phytoplasmas can be present in infected trees and in scion and rootstock material of apple. Natural spread occurs with root anastomosis. Insects (Psyllids) serve as vectors, however, their importance in spread is not assessed. There is no seed or pollen transmission.
Detection and inspection
The presence of clear symptoms (witches’ broom and enlarged stipules) gives strong presumptive evidence of identification. The presence of a fine hairy root on nursery plants during winter may be another indication. A certain diagnosis requires a laboratory examination (e.g. PCR, ELISA) as some of the symptoms can be caused by other factors.
Pest status and importance
This is one of the most important phytoplasma diseases of apple, affecting almost all cultivars, reducing size (by about 50%), weight (by 63-74%), and quality of fruit, as well as reducing tree vigour and increasing susceptibility to powdery mildew (Podosphaera leucotricha).
Source of information
See further information here:
Author: Christiane Scheel and Elise T. Yamamoto Buch
Editor: Dorthe Vestergaard