Tag: plants

Having written about two very large and very important plant diseases, I want today to focus on a much less known fungal pathogen, Rhizoctonia solani.

There is a personal aspect to it. In 1993 I joined the Department of Plant Sciences at the University of Cambridge as a post-doctoral researcher. I already had been working on human disease modelling, but this was the first time I encountered plant pathogens. Colleagues who were biologists and mycologists introduced me to a wonderful world of soil-borne fungi. I have been fascinated with the fungi ever since.

Rhizoctonia is a group of fungi causing various symptoms in plants, including sheath blight (the second-most devastating disease of rice after rice blast) and damping-off which affects seedlings. In the field, it can form densely infected patches causing large yield losses.

As a modeller, I was not allowed to do experiments (in case I do something stupid which will spoil the work), but I enjoyed seeing the experiments set up. We grew Rhizoctonia solani in trays filled with sand in which seedlings of brassicas (radish, cauliflower) were planted.

Another fungus, Trichoderma viride was introduced on poppy seeds, in an attempt to control the disease. A poppy seed had to be placed in an exact position and if dropped by mistake, they would ruin the experiment. It was amazing that the poppy seeds could not really be distinguished from grains of sand which made placing them tricky – but Rhizoctonia knew exactly where they were. A race between Rhizoctonia and Trichoderma ensued to see which one will get to “food” earlier.

We also did field experiments and I really enjoyed going out, meeting farmers and getting my boots dirty. Talking to people who actually grow the plants – propagators, farmers and farm managers – made me realise that mathematical and statistical modelling of plant pests and diseases must be connected to economics and behavioural sciences – a line of research that I have been pursuing since.

A couple of years ago I was writing a paper on how modelling can be used to address the epidemiology aspects of One Health. I was looking for examples when outbreaks of plant pests and pathogens were linked to catastrophic changes in human health. It was then that I became aware of the Bengal Famine of 1942-43.

In 1942 in Bengal, a province of then British India, a fungal infection, Cochliobolus miyabeanus (Brown Spot), was spreading through rice fields. The impact of the disease was intensified by tropical storms on 16-17 October which widely distributed the fungal spores while also killing 14,500 people and destroying fields and rice paddies.

When rice plants were attacked by the fungus, brown patches and discolouration appeared on leaves and stems, and the plants started to die.

The resulting carnage caused estimated yield losses of up to 91% of rice.

Massive starvation followed with the resulting decrease in resistance to diseases. Meanwhile, the weather also created conditions conducive to mosquito breeding leading to an outbreak of malaria. While the first wave of deaths (Winter 1942) was largely caused by starvation, the second wave (1943-44) was dominated by human disease, with malaria, cholera and smallpox thriving in an already affected population. As a result, an estimated 2-3 million people died in a population of 60m.

Even today, rice seedling mortality rate of up to 60% caused by Brown Spot is recorded in some countries and crop yield can be reduced by up to 40%. Prevention is based on disease-free seed and resistant varieties as well as such practices as less dense planting and keeping weeds down. Soil and plant pesticides are also used to fight the disease.

There is an ongoing dispute about the origin and course of the Bengal Famine. The context was the middle of World War 2 with a threat of Japan invasion of British India. But, the British and local government and merchants have been accused of gross mismanagement of food supplies and thus of either causing or not alleviating the famine and death of so many people.

“Though administrative failures were immediately responsible for this human suffering, the principal cause of the short crop production of 1942 was the [plant] epidemic … nothing as devastating … has been recorded in plant pathological literature”.

Padmanabhan SY. The Great Bengal Famine. Annu.Rev.Phytopathol. 1973; 11(1): p. 11-24.doi:10.1146/annurev.py.11.090173.000303

Without going deeply into the political and social causes of and mechanisms for the Famine, the events of 1942-43 show how a plant disease outbreak can become a tipping point and trigger massive suffering.

On a personal note, here is me in India, working with colleagues on plant pest detection and control:

So far my blog has been dominated by coronavirus and its spread and control, but there are other topics that are almost as important for our well-being. This week we are celebrating #PlantHealthWeek, part of the International Year of Plant Health 2020 #IYPH2020. To mark this occasion I hope to write a post each day, to give you, my readers, an idea of how important plants – and plant pests – are to our lives and to mention data and statistics.

Back in the early 1990s, I was a young post-doc in Cambridge who had just started working in the Department of Plant Sciences. Our Common Room (for those not familiar with the University of Cambridge, this is a room where we gathered for our regular tea and coffee breaks) was decorated with portraits of all Botany Professors from 19th century onwards. One painting was showing a rather imposing looking man with a really big moustache, Harry Marshall Ward (1854-1906; Professor of Botany 1895-1906).

In mid-1800s coffee was grown in Ceylon (modern Sri Lanka) and the British plantation owners planted a monoculture of coffee trees on almost every available piece of land. In the 1860s a disease, coffee rust, was spreading in the plantations, killing the trees. Leaves will develop yellow patches, the plant will be unable to produce food and the trees will start to die. The production dropped from 45,000 tons in 1870 to 2,300 tons in 1889.

Mycologists Michael Joseph Berkeley and Christopher Edmund Broome discovered that the disease was caused by a fungus and they named it Hemileia vastatrix – the first part of the name reflecting a half-smooth shape of “spores” i.e. little “seeds” by which the fungus spreads, and “vastatrix” meaning a “she-destroyer”.

Ward came to Ceylon in 1880 to help find the cure for the coffee rust. He recommended avoiding monoculture by growing different types of coffee. He also pioneered “agroforestry” – growing coffee under trees to stop spores being flown by the wind. Growing coffee trees – which are quite small – together with tall trees is practised now as this helps to maintain the right microclimate and shielding from strong sun, as well as providing additional income from timber.

Ward did not manage to solve the Ceylon problem with the coffee rust and the coffee industry there was completely devastated. In order to find a replacement, plantation owners started growing tea which then became the famous Ceylon tea. The English tradition of drinking tea apparently has its origin at that time.

The coffee rust did not stop there and over the years spread throughout the world. There is a constant race between breeding new resistant coffee varieties and the pathogen overcoming the barriers, and between finding new pesticides and the fungus finding ways to resist them. Coffee rust, La roya, is a huge problem across the globe, causing multiple economic and social tragedies.

By the way, I still have not figured out how Professor Ward managed to drink or eat with his moustache…