Ash yellows

Candidatus Phytoplasma fraxini is a species of phytoplasma, a specialized group of bacteria which lack a cell wall and attack the phloem of plants. This phytoplasma causes the diseases ash yellows and lilac witches' broom. Ca. Phytoplasma fraxini was first described in 1999 as the causal agent of ash yellows and lilac witches'-broom. It belongs to the 16 Sr group VII, subgroup A. The pathogen is found in Central and North Eastern USA and Central and Eastern Canada. It was reported from Chile for the first time in 2011. It was then reported from a range of trees in Colombia. In 2017 it was reported infecting grapevine in Iran. Ca. Phytoplasma fraxini can infect a wide range of species of Fraxinus (ash) including: white ash, narrow leaved ash, bunge ash, European ash, Oregon ash, black ash, manna ash, green ash, pumpkin ash, blue ash, Tianshan ash, Urapan and velvet ash. A number of Syringa (lilac) species and hybrids are recorded as hosts including: Syringa x diversifolia, Syringa x henryi, Syringa x josiflexa, Hungarian lilac, nodding lilac, (cut-leaf lilac), Korean lilac, Syringa x nanceiana, Syringa oblata, Persian lilac, Syringa x prestoniae, Syringa pubescens, Chengtu lilac, Syringa tomentella, late lilac, common lilac and Yunnan lilac. In Canada, a few species of Prunus have been recorded as hosts: peach and Pembina plum. This disease is relatively new so the disease cycle is mostly unknown, although it is thought that insect vectors are used as the mode of transmission. The disease is difficult to diagnose because some symptoms match those of stressed environmental conditions such as drought, flooding, or shallow soil. When trees become infected they can survive for many years or immediately die depending on the environmental conditions and health of the tree. The major symptom of this disease is witches broom which causes branches in tufts. Unfortunately, there are not many strategies for controlling the disease besides avoiding areas where it is more prevalent. Ash yellows refers to the disease that occures on ash trees, usually white ash (Fraxinus americana) and green ash (Fraxinus pennsylvanica). All ages and sizes of ashes are vulnerable to infection and symptoms will occur within three years of infection. Some of these symptoms include progressive loss of vitality, subnormal growth, and leaves that fail to reach normal size and are often light green to chlorotic.Additional symptoms include branch dieback, cracks in the bark, early color change in the fall, and premature death of trees. This disease is often diagnosed by the presence of a distinct symptom known as witches broom. Witches broom occurs when there is an overgrowth of the branches of the host, resulting in a clumps of branches that resemble the head of a broom. This usually occurs near the soil line but can occur higher up in the tree as well. Not much is known about the disease cycle of ash yellows. It is unknown how exactly the Phytoplasma enters the tree and how it spreads but insect vectors are suspected. This is due to the fact that Phytoplasmas are often spread in the salivary secretions of insects during feeding. One likely candidate for spreading these Phytoplasmas, also known as Mycoplasma-like organisms (MLOs), is leafhoppers because data shows they are the most common insect vector for MLOs. Once inside the host, the Phytoplasma attacks the tree's vascular system, specifically targeting the phloem sieve tubes. Additionally, the severity of the disease symptoms caused by the Phytoplasma varies. In one study that was conducted, 12 different strains of Phytoplasmas belonging to the ash yellows group were tested and it was found that these strains varied significantly in aggressiveness and the impact they had on growth of the host.