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Foaming at the trunk

This morning, while walking my dog through an old plantation of Monterey pines (Pinus radiata), I noticed that the trees were frothing. The froth looked like soapsuds dripping out of cracks in the bark (see photo). Is this a chemical reaction, or a result of biological action? It was an exceptionally cold first month of winter, and the previous day it had been raining continuously until early afternoon, when it turned out clear and sunny. However, more overnight rain was still falling when the photographs were taken. The foam was only evident on the wetted parts of the bark, and not on dead or older, mature trees.

• This is a disease generally referred to as slime flux. It is usually caused by a low-level fungal or bacterial invasion in a wound on the trunk, and produces a fluid discharge and a lot of froth. Imagine the head on a glass of good stout or lager and you have the idea.

Usually the pathogen is localised: it just sits there and ferments the tree sap, resulting in a breakdown of the tree tissue and producing a waste fluid and copious amounts of gas – presumably including plentiful carbon dioxide. Because the liquid discharge contains sugars, proteins and water, it has a tendency to form bubbles as expelled gases move through it. The flux spills onto the trunk as a frothy, bubbly brew, and is usually accompanied by the smell of fermentation.

Slime flux can be caused by a number of pathogens, including Phytophora, fungi and a host of bacteria. Although not much has been written about yeasts, there is a high likelihood that they are also involved.

Bill Barnes

Consulting Horticulturist

Lorax Farms, Warrington, Pennsylvania,

US

Fasten seatbelts

I recently flew back from holiday on a large airliner. During the flight we encountered severe turbulence. Food and drink went flying, overhead lockers opened, people were screaming and crying, and even the cabin crew were alarmed, crawling along the aisle to take refuge. The plane seemed to fall vertically for about 5 seconds. How much danger were we in? It felt as if the plane was going to fall out of the sky. Has that ever happened?

• The questioner has experienced the effects of clear air turbulence, or CAT. Pilots are unable to see CAT, and it can indeed cause an aircraft to crash, especially if encountered just after take-off or just before landing. Since 1981, there have been more than 350 reports of aircraft running into serious turbulence. It is also considered to be the leading cause of in-flight injuries: in the US, around 60 passengers are injured each year as a result of CAT, which is why passengers are often advised to keep their seatbelts fastened.

There are five major causes of CAT: jet streams; the wake of other aircraft; airflow over mountains; thermals and microbursts; and severe downdrafts associated with rain and thunder clouds. The encounter described here seems to have been caused by jet stream turbulence. On long flights, aircraft try to fly along the jet stream if they can, but since this is usually found at altitudes above 40,000 feet, planes often fly just below it, where there can be turbulent zones.

If an aircraft at cruising altitude encounters a downdraft, the wings lose lift and the aircraft drops suddenly. Anyone or anything not fastened down then hits the ceiling with varying degrees of force, and it is usually the flight attendants who suffer injuries as a result. When the aircraft flies out of the downdraft, the wings regain lift with a loud bang. Aircraft wings are designed to withstand 1.5 g of negative lift and 2.5 g positive – any more than this and damage will occur.

On 5 March 1966 CAT caused a major disaster. On a clear day the pilot of British Overseas Airways Corporation flight 911 decided to make a detour to give his passengers a sightseeing tour of Mount Fuji. The Boeing 707 broke apart as a result of the severe turbulence encountered as the aircraft flew too close to the mountain.

There are several other instances where aircraft have crashed on landing or take-off, often as a result of microbursts. Planes on approach have been flipped over and had insufficient altitude to take corrective action. For this reason, predictive wind-shear radar is now required by law on civil airliners in the US. Such radar can detect the water droplets usually associated with microbursts, and warn the pilot to turn away.

Terence Hollingworth

Blagnac, France

This week’s questions

Whisking disaster

For years, whenever my family have come to visit I have made meringues, which involves whisking egg whites until they are thick. I’ve always used free-range eggs, but recently I bought organic free-range ones, and no matter how much I whisked these whites they would not thicken. Why should organic eggs behave in this way? Is something missing from the birds’ organic diet that prevents the whites of their eggs thickening?

Vera Gaylor

Billericay, Essex, UK

Tricks of the light

In some bars and clubs, the toilet cubicles have an ultraviolet light. I have heard that this is to stop people injecting drugs into their veins. Is this true, and if so, how does it make the veins invisible?

Toby Richardson

Watford, Hertfordshire, UK