March/April 2024 - 7

A blighted walnut
fruit with the infection moving
into the spur and killing buds. Photos
by Themis J. Michailides, Ph.D.
iseases caused by fungi members in the family of
Botryosphaeriaceae (Bot) are characterized mainly
by the development of cankers. These fungi attack
shoots, branches, scaffolds and trunks of trees causing killing
of the plant tissues and forming a canker.
Botryosphaeriaceae fungi are also known for decaying
tropical and subtropical fruit by infecting them in the field
as latent infections and developing in postharvest storage of
these fruit. However, diseases caused by Botryosphaeriaceae
are not well known causing active fruit blight (killing), which
is associated with severe yield losses. Before referring to specific
examples, let us define what is a blight and what is canker.
A blight is the killing of entire vegetative or fruiting plant
tissues without the necessity of the pathogen to colonize all
the blighted tissues. A blight can result in the development
of a canker.
infected pistachio
tree by Botryosphaeria panicle
and shoot blight.
Referring to trees, a canker is a continuous mass of killed
tissues in trunks, scaffolds, branches and shoots where the
canker pathogens has grown and colonized the entire cankered
tissues and beyond. A canker can result in the development of
a blight. However, a blight does not need a canker to occur.
Two good examples of disease representing a canker and a
blight phases are the Botryosphaeria panicle and hoot blight of
pistachio and the Botryosphaeria canker and blight of walnut.
Botryosphaeria panicle
Botryosphaeria panicle and shoot blight is caused by 10
members of the Botryosphaeriaceae fungal family. All these
fungi produce pycnidia, flask-like structures that contain
the spores of these fungi. The spores, otherwise called
pycnidiospores, spread by water (water-splashed) and infect
all the current growth green tissues.
walnut fruit because
of the Bot blight.
During the fall and winter, these spores fall on dormant
vegetative and flower buds and are trapped among the basal
bud scales. Under conducing conditions, the first blighted
structure is entire blighted leaves from infections at the base
of the petiole, or blighted leaflets that can defoliate as well.
Another early blighted tissue can be the young panicles, only if
the temperatures are warm in the spring.
Infections at the base of young developing shoots from
contaminated buds turn black resulting in leaf wilting, and
the blight of the entire shoot. These shoots retain their killed
leaves which will turn to bright brown color very distinct
among the green, healthy foliage. In years with severe disease,
multiple blighted shoots can develop per tree. But the most
devastating blight phase of the disease is the panicle blight (the
killing of the fruit clusters).
The blighting of the pistachio cluster can occur either from
infection of bud or latent infection of the fruit. Infection of
the fruiting bud can lead to girdling of the cluster since the
infection will develop at the rachis (main stem) base, and
it is usually very quick (one week). In contrast, fruit latent
infections will develop during summer and will blight the fruit.
The infection from the fruit will move into the rachis to kill
either part of the cluster or the entire cluster, depending when
the latent infection was triggered to express disease symptoms.
Blight of a pistachio cluster can also occur when large
Hemiptera insects (LFB, Chlorohoa spp., etc.) puncture

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