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Could new varieties present a solution to leaf rust?

The threat posed by leaf rust to coffee farmers across the Americas is hard to overstate.

Indeed, even a cursory glance at either the agricultural or the business press will show the extent of the damage that is being done by the pathogen, which effectively causes an orange dust to settle on coffee plant leaves, ruining entire fields of crops.

According to the Wall Street Journal, Guatemala is set to experience a five per cent drop in its coffee output over the coming 12 months due to the ongoing leaf rust epidemic crossing Central America, while in neighbouring Honduras, the problem will cost growers an estimated $600 million over the same period.

Unsurprisingly, then, tackling the pathogen – which has the scientific name Hemileia vastarix – is a leading concern for growers of all scales and sizes, with a variety of weapons in their arsenal.

First and foremost, many farmers continue to turn to more conventional methods – more specifically, to synthetic fungicides such as Triazaline, developed by scientists with the sole aim of minimising the impact of leaf rust.

However, according to some growers, the solution could lie in growing hybrid plants developed to be resistant to the problem, with advocates of such a path able to draw upon a number of success stories from around the world.

As the Columbian Coffee Hub notes, the objective of improving the genetic base of coffee plants through cross-breeding is to obtain the desirable characteristics of certain plants – that is, their resistance to this pathogen – while not compromising in other areas, most notably when it comes to durability and, above all, bean quality.

With this in mind, growers have spent years breeding a coffee plant found only on Timor Island with a range of commonly-used Arabica plants, with the Genetic Breeding Programme, of which the Hub is a part, pushing ahead with this research through several schemes taking place across Central and South America.

In particular, writing in the pages of Barista Magazine, the Columbian Coffee Hub notes that the Castillo plant – a hybrid of the Timor Island plant and a susceptible Arabica plant – has been faring especially well and is now the flagship coffee bean for the "Colombia without rust" initiative.

Not only is the Castillo plant extra-resistant to leaf rust, growers know they will be getting good-sized beans harvest after harvest – another key reason why this has established itself as one of the region's most popular hybrids.

Moreover, scientists deliberately ensured that they added genetic material from several other plants while making Castillo so as to ensure it is both as resistant as possible and that it can grow in different climates, rather than in just a single region.

"We use seeds from different trees to combine different vaccines for rust resistance, not only one vaccine for a single tree," says Herrando Cortina of the National Centre for Coffee Research (Cenicafe), a partner of the Columbian Coffee Hub.

With leaf rust not just affecting individual growers, but having a wider economic and social impact on communities across countries such as Guatemala and Honduras, the pressure is on both to deal with the current epidemic and to guard against future outbreaks.

While fungicides continue to play a crucial role in the short-term, by slowing and halting outbreaks, new plant varieties could be the best long-term solution, potentially even putting an end to leaf rust once and for all.