Source of pathogen that causes bitter rot disease

Date:
August 14, 2020
Source:
Penn State
Summary:
Fungal spores responsible for bitter rot disease, a common and
devastating infection in fruit, do not encounter their host plants
by chance. Turns out, they have a symbiotic association with the
plant, often living inside its leaves. The new way of looking at
the fungal pathogen, Colletotrichum fioriniae, as a leaf endophyte
-- bacterial or fungal microorganisms that colonize healthy plant
tissue -- was the outcome of a two-year study.



FULL STORY ========================================================================== Fungal spores responsible for bitter rot disease, a common and devastating infection in fruit, do not encounter their host plants by chance. Turns
out, they have a symbiotic association with the plant, often living
inside its leaves.


==========================================================================
The new way of looking at the fungal pathogen, Colletotrichum fioriniae,
as a leaf endophyte -- bacterial or fungal microorganisms that colonize
healthy plant tissue -- was the outcome of a two-year study conducted
by researchers in Penn State's College of Agricultural Sciences.

According to Phillip Martin, a doctoral candidate in plant pathology, the findings, which were published recently in the journal Phytopathology,
have important implications for the management of the pathogen in
fruit trees.

Colletotrichum fioriniae causes diseases, often called anthracnoses,
in more than 100 fruit and vegetable plants, including apple, peach,
pear and strawberry. The fungus infects the fruit under warm and wet
conditions and causes brown, sunken lesions; occasionally, orange spores
will be seen on the surface.

The disease is of concern to the Pennsylvania apple industry, which
produces 400 million to 500 million pounds of apples per year. The state
ranks fourth in the nation for apple production, per statistics from
the U.S. Department of Agriculture.

"The research was based on the idea that if we can determine where
the spores are coming from, then maybe we can eliminate the source and
break the bitter rot disease cycle," said Martin, who carried out the
study under the guidance of Kari Peter, associate research professor of tree-fruit pathology.

"Unfortunately, from this perspective, many of the spores come from
leaves, including apple leaves, and from trees and shrubs that are
everywhere in Pennsylvania." Previously, the spores in question were
thought to originate mostly from diseased fruits and twigs. However,
even when infected fruits and twigs were removed from a tree, the
disease, while reduced, often still was present, a circumstance that
puzzled scientists.



==========================================================================
The research, which took place in 2018 and 2019, focused on apples
and involved the placement of rain-splash spore traps in orchards
at Penn State's Fruit Research and Extension Center, at Hollabaugh
Bros. Inc. fruit and vegetable farm, and at a satellite location in Arendtsville, all of which are located in Adams County. Traps also were
placed in two forested areas -- comprised mostly of deciduous trees --
near the orchards.

Based on previous research that indicated that Colletotrichum fioriniae
could survive on leaves, the team collected more than 1,000 leaves of
apple and of 24 forest plant species. The leaves were disinfected to
kill fungi on the leaf surface, frozen to kill the leaves and incubated
to allow the fungi inside of the leaves to grow out and sporulate.

This test found Colletotrichum fioriniae in more than 30% of leaves
sampled, with most spores coming from the forest samples. In orchards
that were managed with fungicides, up to 8% of apple leaves were infected
with the fungus. In the untreated orchard, Martin said, the spores were abundant, meaning they were found in 15-80% of the leaves. The infections
did not seem to be causing any leaf diseases, however.

"While unexpected, these findings did explain why growers struggle with
bitter rot even when they remove all diseased fruits and twigs -- the
fungus was living in the leaves during the season," Martin said. "The
fungus was present in all the tested orchards and could not be traced to infection from a nursery, which makes sense since the initial infections
likely are coming from surrounding forests and fence rows." Since the
fungus is abundant in the forest canopy, eradication from nearby areas
would be impractical, Martin added. However, the spatial limitations
of rain-splash dispersal mean that forests are not regular sources of
fungus spread; they likely serve only as primary introduction sources
during extreme rain and wind events, after which the fungus becomes
established in agricultural areas.

"Our study changes how we think about this fungus," Martin said. "While
it may not supply quick fixes, it provides the basis for further
research aimed at developing better management techniques, such as
selecting resistant cultivars and breeding for genetic resistance."
Peter agreed. "Although it's exciting to understand that Colletotrichum fiorinae's niche in the environment is more sophisticated than we had appreciated, it does make managing bitter rot in apple orchards less straightforward," she said. "As researchers, we can view this is an
opportunity to think outside the box and to be creative in figuring
out a sustainable bitter rot management strategy." In the meantime,
Martin noted, disease-management tactics stay the same. "We don't believe
most spores are overwintering in the leaves," he said. "Growers should
continue to remove the infected fruits and twigs to help reduce disease
spread season to season."

========================================================================== Story Source: Materials provided by Penn_State. Original written by Amy
Duke. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Phillip L. Martin, Kari Peter. Quantification of Colletotrichum
fioriniae
in orchards and deciduous forests indicates it is primarily a leaf
endophyte. Phytopathology(R), 2020; DOI: 10.1094/PHYTO-05-20-0157-R ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200814131008.htm

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