September 2022 - 14

POSTHARVEST
Guidelines for control of speck rot in apples
By Chang-Lin Xiao
USDA-ARS, San Joaquin Valley
Agricultural Sciences Center, Parlier CA
Speck rot, a postharvest fruit rot
disease of apple, is caused by the
pathogenic fungus Phacidiopycnis
washingtonensis. Fruit infection
originates in the orchard.
The source of the inoculum comes
from dead or diseased plant tissue
of the " Manchurian " crabapple
pollinizers in affected orchards.
The fungus produces small black
dots (fruiting bodies = pycnidia)
on Manchurian twigs, tree branches
and crabapple fruit. Fruiting bodies
contain millions of infective spores
that can be spread by rain, irrigation
or over-tree cooling to nearby apple
trees and fruit.
Although apple fruit infection
occurs in the orchard, fruit rot
symptoms develop postharvest,
during storage or at the market.
The cankers and twig dieback
caused by P. washingtonensis
are not common on apple trees
in commercial apple orchards in
Washington State, but the Manchurian
crabapple pollinizer trees are highly
susceptible. Detailed pruning of this
pollinizer is strongly recommended
to significantly reduce the infective
potential in commercial orchards.
Orchard symptoms
The pathogen is associated with
dead tissues on apple trees. It can also
cause twig dieback and canker disease
of the crabapples used as pollinators.
P. washingtonensis is a weak canker
pathogen to apple trees. The fungus
causes small black dots (fruiting
Speck rot can also infect crabapple fruit and form fruiting bodies on the mummified fruit left hanging on trees. Photo: R. Kim,
Pace International
bodies) to form on infected twigs
and tree branches. Fruiting bodies
contain millions of spores which serve
as inoculum for fruit infection. The
fungal spores are then spread by rain,
irrigation water or over- tree cooling.
Postharvest fruit symptoms
Speck rot symptoms can also occur
as either a stem-end rot, as a calyxend
rot, or both. The affected fruit
tissue is spongy to firm, which is not
differentiable from gray mold and not
readily separable from the healthy
tissue. The color of the decayed
area varies from light brown to dark
brown, or occasionally black. Speck
rot is so named because the brown
to black specks with white to light
tan centers that may appear around
the lenticels, especially on red apple
cultivars.
Orchard disease management
Because the major source of
inoculum comes from infected
crabapple (both dead tissues of trees
and fruit mummies), pruning is highly
recommended to reduce infection by
P. washingtonensis. Detailed pruning
to remove all infected twigs, branches
and fruit mummies will significantly
reduce infection by these pathogens.
Any approach to pruning that
still leaves a considerable amount
of infected tissues should be
reconsidered. However, any amount of
pruning is preferable to leaving trees
unpruned. A video on the preferred
method of pruning for pathogen
management can be found here:
http://treefruit.wsu.edu/videos
Water spreads the fungal inoculum
and creates conditions conducive
for fruit infection. Therefore, it is
recommended that overhead irrigation
be avoided, and that over-tree cooling
be limited in duration to only the
amount necessary for sunburn
prevention. Preharvest fungicides such
as Ziram, Topsin M or Pristine applied
14 | FruitGrowersNews.com

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September 2022

Table of Contents for the Digital Edition of September 2022