We're just over three months into
UMF’s experiment comparing chemical and organic treatments for
coffee leaf rust. The experiment is going well, but full details on
how the treatments are working will come later, as we are still
analyzing the most recent sampling. Apart from the treatment results,
we have observed something interesting and unexpected. We first saw
it a little over a month ago, and we confirmed it with formal
sampling this past week: branches that don’t have any leaves start
to die, but branches with even one leaf remaining, stay alive.
In our analysis, we found that 91% of
branches that have no leaves showed signs of dying (dieback):
blackened ends and softening of the branch. However, only 16% of
branches with a single leaf showed signs of dieback. And 0% of
branches that had two or more leaves showed signs of dieback.
These results are interesting for two
reasons. First, it suggests that keeping a single leaf on a branch
will prevent most long-term damage that will affect future harvests.
Having two leaves on a branch ensures that no long-term damage will
occur to the branch.
For farmers dealing with leaf rust,
this observation has important potential implications. Throughout the
course of the harvest year, a coffee plant grows new leaves, although
this growth is concentrated during the rainy season. Farmers may be
able to prevent long-term damage to a plant simply by using proper
preventative measures to protect new leaves, even when older leaves
of a plant are highly infected by leaf rust.
Second, our observation suggests that
the long-term effects of leaf rust are branch-based, rather than
plant-based. If a branch losing all of its leaves had a negative
effect on the rest of the plant, we would expect to see a larger
number of branches that show dieback but still have leaves. This
means that proper treatment of leaf rust on one branch will not
necessarily prevent long-term damage to other branches on that same
tree. But, conversely, damage to one branch will not necessarily
cause damage to another branch.
Furthermore, it
appears that the effects of leaf rust are not transferred between
primary branches and the secondary branches that grow off of them.
Even though they are part of the same branch, we are seeing that
secondary branches can lose all of their leaves and start to die
while the primary branch continues to be healthy, and vice versa.
These numbers are not yet conclusive,
and further experiments will need to be run in the future to be
certain that leaf loss is causing, not just associated, with the
dying of branches. There is still much to be done in terms of
understanding how to take these new results and convert them into
practical leaf rust prevention advice to farmers. However, they are
certainly suggestive that the last few leaves are vital to the
survival of coffee plants. We will continue to keep you informed as
the experiment progresses.