March/April 2024 - 9

rain these structures will swell, increasing the pressure in
the pseudothecium and the asci that will shout the spores
(ascospores) in the air.
In contrast with the Bot disease of pistachio, for Bot of
walnut there are two types of inoculum that overwinter on
the trees and/or the ground in pruned shoots and branches
left in the orchard. For both diseases, the cankers extend
internally in the tissues about 2 inches beyond the external
margins of the canker.
When suggestions are given to growers to prune
the cankers and reduce inoculum in the orchard, the
recommendation is always to prune about 2 to 3 inches
beyond the external canker margin.
Particularly in walnuts, pruning wounds are very
susceptible to infection by Botryosphaeria fungi in contrast
to pruning wounds of pistachio. In walnuts, the shoots have
a central channel which is filled with loose pith. When it
rains, the pith acts as a sponge absorbing and holding the
water longer, thus giving a chance for the spores that have
landed on the pruning wound to germinate and establish
an infection.
Using artificial inoculation experiments, we showed
that pruning wounds of walnut can remain susceptible to
infection for at least four months. In addition, the presence
of walnut scale (Quadiaspidiotus juglandis) weakens the shoot
and infections occur more frequently, resulting in blighted
shoots and branches. Also in walnuts, lower limbs that are
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shaded eventually die, are colonized by walnut scale and
covered with Botryosphaeria fungi.
This is because the Botryosphaeria fungi also exhibit
an endophytic phase. An endophyte, according to the
definition of the American Phytopathological Society, is a
close association and reproduction of the fungus and the
host tissues, without the expression of any disease symptoms
(lesion, blight or canker).
Control of Bot of walnut is achieved with pruning major
branches to reduce inoculum, and fungicide sprays starting
in mid-May, mid-June, and mid-July. Sprays applied after
late July are not effective (
fungicideefficacytiming.pdf). Sprays applied just before
and during early hull split are effective in reducing mold
of walnuts, which is caused mainly by Alternaria and
Fusarium spp.
In orchards where Bot canker and blight occurs, part of the
walnut mold is caused by Botryosphaeria. The blight phase of
Bot diseases can cause more yield reduction than the canker
phase. Therefore, growers need to preventatively protect
plant tissues from infections occurring.
Themis J. Michailides, Ph.D., Department of Plant Pathology,
University of California, Davis/Kearney Agricultural Research
and Extension Center, Parlier, California. You may reach
Michailides via email or by phone at

March/April 2024

Table of Contents for the Digital Edition of March/April 2024

Editor's Letter
The blight phase of Botryosphaeria diseases in nut crops of California
Tree nut outlook for 2024
From the top
Water stress
Buzz pollination
Hazelnut crop concerns
Almond Board of California
Ad Index
March/April 2024 - 1
March/April 2024 - 2
March/April 2024 - 3
March/April 2024 - Editor's Letter
March/April 2024 - 5
March/April 2024 - The blight phase of Botryosphaeria diseases in nut crops of California
March/April 2024 - 7
March/April 2024 - 8
March/April 2024 - 9
March/April 2024 - Tree nut outlook for 2024
March/April 2024 - 11
March/April 2024 - 12
March/April 2024 - 13
March/April 2024 - From the top
March/April 2024 - 15
March/April 2024 - 16
March/April 2024 - 17
March/April 2024 - Water stress
March/April 2024 - 19
March/April 2024 - Buzz pollination
March/April 2024 - 21
March/April 2024 - Hazelnut crop concerns
March/April 2024 - 23
March/April 2024 - Business
March/April 2024 - 25
March/April 2024 - Almond Board of California
March/April 2024 - Events
March/April 2024 - 28