BSPP ‘Plant Doctor’ Quiz #11 – February 2022 by Eric Boa

The correct answer is:

Squirrel damage. They strip the bark to feed on the sweet sap below. Once the stem is ringed the leaves above die quickly. Frequently seen in some years on isolated or small groups of trees.

Comments on the other options:

The large area with dying leaves suggests that a major branch has been damaged below. Wilt pathogens result in a more uneven pattern of leaf death; and I don’t know of any such pathogens affecting sycamore, or Acer species in the UK. The tree was too far from the sea to be affected by salt damage, and the pattern of damage would have been less severe and more diffuse.

The correct answer is:

Bacterial disease. Pseudomonas savastanoi pv. fraxini.

Comments on the other options:

‘Genetic abnormality’ might appear an attractive option yet these are distinctive symptoms with a scattered appearance, more indicative of a precise cause. Insects and mites are common gall-formers on leaves and fleshy tissue – not on woody stems. Yes, there is a fungal canker on ash (Neonectria ditissima), which kills tissue rather than stimulating new growth, as seen here. P. savastanoi is an unusual and relatively unknown pathogen, yet widespread and damaging on olive trees (related to ash).

The correct answer is:

Fire damage. This symptom really confused me when I carried out a survey of neem pests and diseases in Niger and Nigeria in the 1990s. Yet looking at afresh it is much clearer that someone has lit a fire below.

Comments on the other options:

Stem borers could cause such symptoms, but not here. The leaves have died suddenly and in a defined area – not an indication of a wilt pathogen. High winds can damage leaves, with the margins drying up and browning. Symptoms would be less severe and more diffuse in a larger area of the crown.

The correct answer is:

Physiological stress (I think). The staining around the exit of this spectacular oozing might suggest something pathological, yet on balance I think this is abiotic. The crowns were healthy and the trees productive, giving further weight to my rapid field diagnosis (I was on holiday).

Comments on the other options:

Fungal gummosis is common on citrus species and is associated with Phytophthora species (I know, it’s not a fungus – but no one calls this Oomycete or Fungus-like gummosis ). The gummosis is lower down on the trunk and accompanied by heavy staining below and dieback and decline in the crown. Gummosis is a common symptom in trees and though pathogens are involved it’s often a sign of physiological disturbance.

The correct answer is:

Leaf miner. The holm oaks around where in live in SW London are plastered with infestations. Doesn’t seem to affect the growth and health of the trees, There are two species found on holm oak. This appears to be Phyllonorycter messaniella, the leaf blotch leafminer.

Comments on the other options:

These are superficial symptoms caused by caterpillars feeding beneath the leaf epidermis. A fungal leaf spot would kill the part of the leaf affected and, typically, have a more defined shape. This evergreen oak has evolved to keep its leaves over winter and though low temperatures could kill off young growth in early spring, for example, it wouldn’t produce these blotches. Nor would a virus.

The correct answer is:

Phytoplasma – maybe. Similar symptoms have been associated with witches’ brooms on Salix spp. in Central Asia, Iran. I’ve seen this in Ukraine and Poland and I expect it to be more widespread yet under-recorded.

Comments on the other options:

There’s no diagnostic evidence to link this with phytoplasma in the UK, yet there’s a clear similarity to symptoms widespread on willows elsewhere. It’s too distinctive and extensive to be a ‘genetic abnormability, an otherwise attractive option for unusual growths. But also consider what insect feeding (not here, in my opinion) and bacterial infections can induce (see ash example earlier). Waterlogging? Nuh!

The correct answer is:

Salt damage. Winters are cold and icy, hence the liberal application of salt on the roads.

Comments on the other options:

Initially confused by the uniform development of symptoms, Dr Phil suggested that salt had been translocated to the leaves, where high concentrations had resulted in this browning. Pests and diseases would produce more varied and scattered symptoms, with evidence of feeding damage, necrosis and so on. High temperatures for prolonged periods are more likely to induce leaf curling prior to death.

The correct answer is:

Fungal (Hymenoscyphus fraxineus) and abiotic dieback. Let’s not forget that ash dieback of the abiotic variety has been around for decades in th UK. Separate the wispy dieback (fungal) around the crown periphery from the larger dead branches I associate with root disturbance and other abiotic factors. Described as stag-headed because they look like antlers.

Comments on the other options:

Air pollution is one of those fall-back options: when pest and disease options have been ostensibly eliminated look for something that could have a widespread impact on tree growth. Nutrient deficiencies is another fall-back option. I’ve not included lightning damage. I said diagnosing tree health problems wasn’t going to be easy …

The correct answer is:

Virus-like particles. Closely associated with this disease, known as Mundulla Yellows yellows. But there are other theories, including poor soils and herbicide damage.

Comments on the other options:

I’m going to stick with the virus-like particles option, partly because the symptoms have that sort of feel: yellowing, unusual growth patterns. Opinion is however divided in the absence of what some say is conclusive proof. Xylella fastidiosa has a huge host range and is a remote possibility – I took this photo from some distance away so didn’t have a chance to examine the leaves. The herbicide damage theory gains little support from this site.

The correct answer is:

Taphrina betulina. Yes, fungi can cause witches’ brooms and other strange growths, such as peach leaf curl (also a Taphrina sp.) and – my favourite – mango malformation (Fusarium!).

Comments on the other options:

Too many, wrong shape and too densely packed to be birds’ nests (says a self-confessed non-birder). Yes, phytoplasmas do frequently induce witches’ brooms, but not on birch. Mistletoes are green, are more regular in shape and less dense.