April 2019 - 22

... continued on from 19
PLANT HEALTH
Treatment
Aliette
Camelot O
Cease
CuPro
EcoSwing
Kalmor
KleenGrow
Kocide 3000
Phyton 27
Regalia
Stargus
Triathlon BA
ZeroTol
x
x
x
x
x
x
x
x
OMRI
FRAC
33
M01
44
M01
UNK
M01
NC
M01
M01
PO5
44
44
NC
Rate/100 gal
12.8 oz
256 oz
256 oz
2 lb
32 oz
32 oz
12 oz
32 oz
20 oz
128 oz
128 oz
128-192 oz
128 oz
Plant
Lilac
Lilac/Hibiscus/Blueberry/Begonia/Tomato
Hibiscus/Tomato/Ficus
Impatiens/Mum/Geranium
Tomato/Hibiscus
Hibiscus/Blueberry
Result
Some
Some/very good
Some/good
Good/excellent
None/very good
Good/excellent
Poinsettia/Lilac/Mum/Blueberry/Tomato/Begonia/Ficus None/excellent
Hibiscus/Blueberry/Tomato/Begonia
Poinsettia/Begonia/Tomato/Ficus/Mandevilla
Impatiens/Lilac/Mandevilla/Ficus/Poinsettia
Mum/Geranium
Mum/Geranium/Hibiscus/Tomato/Ficus
Impatiens/Lilac/Mum/Geranium/Ficus/Poinsettia
Some/very good
None/excellent
None/very good
Good
Some/good
None/very good
Table 2. Efficacy of bactericides on ornamentals and other greenhouse crops with Pseudomonas or Xanthomonas leaf spot (2012-2018).
bacterial pathogens or fungal pathogen or both. Table 1 shows the most
common characteristics for phytotoxicity or fungal spots compared to
those caused by bacteria.
The only reason you would want to know which bacterium is involved
is to make sure it is the cause of the symptom and then to consider all
possible hosts in a spray program. Otherwise control strategies will be
nearly identical for one bacterial disease or another.
One last point on bacterial diagnosis: just because you get a lab report
that says a specific bacterium is the cause of your problem, it does not
prove it is the cause. If it is a common disease then you are set. However,
if it is new, ask for feedback from an expert from the university Extension
service whenever possible.
CULTURAL CONTROLS
Generally, using cultural controls is not as effective for bacterial leaf
spot diseases as for some other diseases like Botrytis blight and downy
mildew. This is mainly due to the fact that the most important cultural
control for foliar bacterial diseases is elimination of overhead watering and
exposure to rainfall. Eliminating water on leaves will completely stop foliar
bacteria from infecting. Since exposure to winter, spring or summer rainfall
(depending on where you are in the U.S.) and use of overhead irrigation is
so common in many operations this cultural control is rarely practical.
If you are producing crops from seed, you need to know that pathogenic
bacteria like Xanthomonas (and also fungi like Alternaria) can be present
in very, very low numbers. Zinnia is the best example of this and treating
the seed with bleach has been very effective for over 30 years. The same
is true for Xanthomonas diseases on stock (Matthiola), cabbage and other
crucifers. Vegetable seeds like tomato and pepper also share at least one
species of Xanthomonas and their seed is often pre-treated/cleaned to
remove the pathogen from the outside of the seed. This type of treatment
has not been routinely done for ornamental disease more or less due to
the high value of the seed and the very small size of our industry.
22 APRIL 2019 GPNMAG.COM
RECENT DISEASE OUTBREAKS
In 2017, an outbreak of bacterial leaf stripe on canna was found in
several nurseries in Florida (canna 'Cannova Rose' and 'Cannova Bronze
Scarlet') and Texas (mixed colors of Cannova). Symptoms included darkbrown
or black spots with a yellow halo that coalesced into necrotic
stripes along leaf veins, especially on immature leaves (Figure 4).
Acidovorax avenae subsp. avenae was identified as the cause of this new
disease. Oregon State University reported the same pathogen on canna
'South Pacific Scarlet' from Washington State (2014). Leaf stripe was also
recently reported causing leaf stripe on Strelitzia nicolai in Florida.
Begonias were first reported infected with Xanthomonas axonopodis
pv. begonia (current name) in 1928 in Denmark. By 1938 it was well
established in Europe and in 1939 the disease was reported in the United
States. Since that time, Xanthomonas leaf spot and blight on begonias has
been reported from nearly every region with occasional outbreaks. Despite
some local folklore, all begonias (including Rex) are susceptible to this
disease (Figure 5).
In late 2017 and early 2018, an outbreak of Xanthomonas blight on
begonia occurred. Since little work (if any) has been reported on this
disease since the late 1980s, we started trials comparing bactericides for
safety and efficacy on begonias infected with Xanthomonas. The crops we
used were: Begonia boliviensis 'Bonfire Orange', Begonia semperflorenscultorum
'Prelude Scarlet', and Begonia tuber-hybrida 'Nonstop Red'.
Treatments were applied three times on a weekly interval and plants were
inoculated after the first treatment. The treatments included:
A. Noninoculated control
B. Inoculated control
C. KleenGrow (6 oz/100 gal)
D. KleenGrow (12 oz/100 gal)
E. Kocide 3000 (32 oz/100 gal)
F. Camelot O (1 percent)
G. Phyton 27 (20 oz/100 gal)

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April 2019

Table of Contents for the Digital Edition of April 2019