PCC March 2019

Photo 1. Anthracnose infection symptoms on
almond hull. Photos courteys of B. Holtz, University
of California Cooperative Extension.
The Many Possible
Causes of
“Gummy Nuts”
in Almonds
By: Emily J. Symmes | Sacramento Valley
Area IPM Advisor University of California
Cooperative Extension and Statewide IPM
Program
Beginning in March, there are a
number of biological factors and
non-biological conditions that
may cause “gumming” in almonds.
When we refer to “gummy nuts”, we are
actually referring to a suite of different
symptoms depending on which part of
the fruit is affected. To distinguish those
in this article, the term “hull gummosis”
refers to the exudates (typically clear to
amber in color) that are visible on the
outside of the fruit. Depending on the
cause and extent of fruit damage, we
may also observe gumming evident on
the shell and in the kernel, in addition
to other symptoms.
In order to develop the best approach
to maintaining crop health and kernel
quality, it is important to be able to
distinguish among the various causes
of hull, shell, and kernel gumming.
In general, dissection of the fruit, as
well as a holistic approach to assessing
the orchard is necessary to determine
the cause of symptom expression and
potential for significant crop damage.
Often, once hull gummosis and possible
underlying kernel damage are observed,
the window of opportunity or ideal
treatment timing to prevent or mitigate
the cause of already-damaged fruit has
passed. However, understanding how
to best identify the cause of symptoms,
relationships to other orchard
factors that lead to the development
of symptoms and damage, and the
timing of initial onset will allow you to
develop improved monitoring and crop
protection strategies.
Diseases
With most hull gummosis arising from
disease infection, there will typically
be other signs and disease symptoms
apparent on other parts of the plant.
Additional factors such as orchard
history of disease, timing of symptom
development, cultivars affected,
and weather patterns conducive to
particular disease development can help
distinguish the cause(s) of symptoms
being observed in the orchard. The most
common pathogen-induced infections
that can lead to hull gummosis include
anthracnose and bacterial spot.
Anthracnose
When small nuts are infected, they
shrivel and turn a rusty orange color.
When larger nuts are infected, they
exhibit round, sunken, orangish lesions
and profuse hull gummosis as the
infection progresses into the kernel.
Anthracnose gummosis is typically
amber in color with multiple exudate
sites on the hull (Photo 1). Eventually,
infected nuts die and remain attached to
the spur as mummies.
Other signs and symptoms of
anthracnose infection may be evident
on flowers, foliage, spurs, shoots,
limbs, and branches. Blossom blight
of infected flowers looks similar to
brown rot strikes. Infected leaves tend
to develop water-soaked lesions that
eventually fade in color, developing into
marginal necrosis. Leaves die but often
remain attached to branches. Dieback
often occurs on shoots and branches
that bear infected nuts.
Prolonged warm (>59°F), rainy weather,
especially extending well into spring,
are most conducive to disease spread
and development. Summer infections
can occur if irrigation contacts the tree
canopy. All varieties are susceptible,
with Butte, Fritz, Monterey, Peerless,
Price, and Winters among the most
susceptible, and Nonpareil considered
less susceptible to anthracnose
infection. Symptom development
on the outside of the fruit (i.e., hull
gummosis) is dependent on the timing
of initial infection and rate of disease
progression. In most years, it is evident
by mid- to late-April.
Bacterial Spot
The most obvious symptoms of
infection occur on nuts. Typically, hull
lesions begin as small water-soaked
circular spots. Infections enlarge and
become necrotic, with obvious lesions
Continued on Page 36
34 Progressive Crop Consultant March/April 2019