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A leafspot and leaf blight caused by the fungus Tubakia dryina (formerly
Actinopelte dryina) have become common on several species of oak (Quercus spp.)
and other landscape ornamentals. There is little information on the biology of
this fungus, but observations indicate that conidia are produced on damaged
leaves and spread by splashing water or rain to infect young leaves as they
emerge and expand. Thus, overhead irrigation or abundant rainfall are favorable
for disease development. Observations in Florida nurseries have shown that the
fungus is capable of producing conidia and infecting trees throughout the growing
season, but disease progress is greater in spring and summer months.
The disease is widely distributed on wild oaks and other woody plants and
commonly occurs on trees in the landscape. In nurseries, irrigated, container-
grown trees are particularly vulnerable to severe damage and defoliation. In
addition, growth is slowed and appearance is degraded. Swamp Laurel oak, Quercus
laurifolia Michx., Laurel oak, Q. hemisphaerica Bartr. or Shumard Red Oak, Q.
shumardii Buckl. are particularly susceptible.
Chemical control recommendations have included applications of copper
fungicides timed to protect new foliage. However, oaks commonly grown for
southern landscapes may produce several flushes of new growth throughout the
growing season and multiple sprays are required. Moreover, variation in growth
among plants produces young and susceptible foliage throughout the growing
season. These considerations may explain why copper and other sprays have been
unsatisfactory for disease suppression. Fungicides with systemic activity would
require less critical timing and could provide better disease suppression in
container-grown or young trees.
MATERIALS AND METHODS
Four fungicides with varying levels of systemic activity, two non-systemic
fungicides, and a copper fungicide were applied to 1-year old Laurel oak trees
'Plant Pathologist, University of Florida, IFAS, Central Florida Research and
Education Center, Sanford, FL 32771.
Commercial companies or trade names are mentioned in this publication solely for the purpose
of providing specific information. Mention of a company or trade names does not constitute
a guarantee or warranty of its products jy the Institute of Food & Agricultural Sciences or
an endorsement over products of other 'companies not mentioned. This publication reports
Research involving pesticides. It does not contain recommendations for their use, nor does
it imply that the uses discussed here have been registered. All uses of pesticides must be
registered by appropriate State and Federal: agencies before they can be recommended.
University of Florida:
Institute of Food & Agricultural Sciences
CENTRAL FLORIDA RESEARCH & EDUCATION CENTER
:h Report SAN 90-06 December 1989
CONTROL OF TUBAKIA DRYINA ON LAUREL OAK WITH SYSTEMIC FUNGICIDES
J. 0. Strandberg1 ;:
growing in .3 gal. containers. Trees were irrigated nightly with overhead
sprinklers. Fungicides and rates were as follows: propiconazole 8E (Tilt),
8 oz/100 gal/acre; difenoconazole 35E (CGA 169374), 100 g a.i./100 gal;
thiabendazole 50% F (Mertect), 8 oz/100 gal, triadimefon 50 WP (Bayleton), 1.0
lb/100 gal; copper hydroxide 40% (Kocide), 2.0 lb/100 gal; and chlorothalonil
50% F (Daconil), 2 pt/100 gal. Treatments were applied at two-week intervals
from May 17 to July 25.
On June 14 and July 25, subjective disease damage ratings were made with
the following rating system: 0 no disease damage; 1- small lesions (< 1 mm)
in moderate numbers; 2.- abundant small lesions on older leaves with occasional
larger necrotic areas; 3 abundant small lesions on all leaves, necrotic areas
on most leaves, some leaves killed; 4 as in #3, except that small branches
were killed, moderate defoliation present and new growth was inhibited. Percent
leaf area damaged (LAD) and stem diameters were obtained on the same dates.
Measurements on June 14 showed that all fungicides reduced disease damage
when compared with unsprayed trees (Table 1). LAD estimates confirmed this, but
indicated that chlorothalonil, was only marginally effective (Table 1). By July
25, differences in disease damage between treatments were greater and disease
damage in all treatments was much less than in sprayed trees. Between the two
sampling dates, disease damage increased in all treatments except where
propiconazole and difenoconazole were applied; in these treatments, neither
damage ratings nor LAD values changed significantly (Table 1). On both sampling
dates, LAD values were well correlated with disease damage ratings.
Plant height and stem diameter were significantly affected by levels of
disease damage resulting from fungicide treatments (Table 2). All treatments
resulted in significant increases in plant height and stem diameter compared-
with untreated plants (Table 2). Treatment with difenoconazole resulted in the
largest stem diameter and plant height (Table 2). Although propiconazole
greatly reduced foliar disease damage and produced an increased stem diameter,
this treatment significantly reduced plant height. The reduction in plant
height was probably a growth response. Plant height and stem diameter were well
correlated with disease ratings and LAD values.
Based on foliar disease damage ratings and LAD measurements, only
difenoconazole and propiconazole among the fungicides tested reduced disease
damage to levels acceptable for containerized nursery tree production.
Thiabendazole and copper hydroxide may be useful under conditions less favorable
to disease development of when applied more frequently. However, all fungicide
treatments reduced disease damage compared to unsprayed plants.
IMPORTANT Of the fungicides tested in this study, only copper hydroxide
is presently registered for use on woody ornamentals. It is presently illegal
to use'other fungicides mentioned in this report on woody ornamentals. Results
reported here indicate that some of the newer systemic fungicides may provide
acceptable disease control when registered for this use.
Table 1. Disease damage ratings and measured percent leaf area damaged (LAD)
on new foliage of 1-yr old Laurel oak trees infected with Tubaki
dryina for'different fungicide treatments.
Fungicide June 14 July 25
treatment Rating1 % LAD1 RatingI % LADI
Propiconazole 1.8 cd 6.4 cd 1.6 d 5.1 d
Difenoconazole 0.8 e 4.8 d 0.5 e 4.6 d
Thiabendazole 0.6 e 10.5 bc 2.1 d 21.8 c
Triademefon 2.3 bc 9.3 cd 3.3 bc 33.2 b
Copper hydroxide 1.6 d 14.7 b 2.7 c 24.4 bc
Chlorothalonil 2.5 b 20.3 a 3..4 b 31.8 b
Control 3.5 a 17.6 a 4.3 a 51.4 a
F value 27.2** 14.1** 39.4** 27.6**
Table 2. Height and stem diameter of 1-yr old Laurel
oak trees infected with Tubakia dryina
following fungicide treatments.
Fungicide Stem diameter1 Plant height1
treatment (cm) (cm)
Propiconazole 1.09 b 79.6 c
Difenoconazole 1.27 a 107.5 a
Thiabendazole 1.01 bc 98.7 b
Triademefon 1.05 bc 79.8 c
Copper hydroxide 0.99 cd 79.0 cd
Chlorothalonil 0.92 d 75.6 d
Control 0.72 e 65.4 e
F value 33.90** 112.34**
1Values are means from four replicates. Means
are not significantly different; Duncan's New
followed by the same letter
Multiple Range Test (P-0.05).