University of Florida
Institute of Food and Agricultural Sciences
CENTRAL FLORIDA RESEARCH AND EDUCATION CENTER
EFFECTS OF LIGHT INTENSITY PROVIDED BY DIFFERENT LEVELS OF SHADE
FABRIC ON THE DEVELOPMENT OF FERN ANTHRACNOSE
James O. Strandberg1
RESEARCH REPORT SAN 99-07
SUMMARY: The effect of light intensity on the development of leatherleaf fern
anthracnose caused by Colletotrichum acutatum was investigated by growing diseased
leatherleaf fern under different levels of light intensity created by shade fabric coverings
that provided 30,40, 50, 60, 73 and 80 percent shade. Disease incidence and damage
was highest under 30 and 40 percent shade. Leatherleaf fern plants grew very poorly
under the higher light intensity levels. Fern growth was best under 73 or 80 percent
shade. There were no apparent differences in anthracnose incidence and damage
levels under 50 to 80 percent shade.
Fern growers have often reported that
fern anthracnose is more severe and
difficult to control in areas where
abundant sunlight reaches fern foliage
for part of the day. The effect is most
common on ferns grown under natural
shade provided by native oak trees
where light intensity is often not uniform.
Sunny areas caused by the damage,
death, or unfavorable spacing of shade
trees are affected most. To investigate
this problem, the effects of light intensity
on anthracnose development was
studied by growing leatherleaf fern
under different light intensity levels
provided by several gradations of shade
fabric and monitoring anthracnose
development and damage.
Figure 1. Percent of leaves infected when grown under 1
= 30%, 2 = 40%, 3 = 50%, 4 = 60%, 5 = 73%, and 6 =
1Professor (Plant Pathology)
Light intensity also affects the growth and development of leatherleaf fern and leaf initiation
and growth greatly affect anthracnose development, so fern leaf initiation and development
were also measured in this experiment.
Leatherleaf fern plants were grown in large plastic pots (21 in. diam x 12.6 in. depth)
containing 2.5 ft3 of a 60% pine bark: 20% peat: 20% sand potting mix amended with
dolomite (0.25 Ib.ft-3) and micro-elements. Three fern plants which had been growing in
6-inch pots for 2 months prior to the experiment were transplanted into each of the large
pots. These plants were infected with anthracnose, and many of the leaves were
severely damaged by disease. To produce a more uniform incidence of disease, all of
the large pots were uniformly inoculated with C. acutatum by spraying them with a
suspension of spores (6800 spores/ml) on December 28 (day 14 of the experiment).
Following inoculation, most of the new leaves that developed continued to become
infected and damaged by the anthracnose pathogen. The pots were placed under
canopies made with polypropylene shade fabric that provided either 30,40, 50, 60, 73 or
80 percent shade and watered as needed. A 20-20-20 soluble fertilizer (approximately
0.06 oz. of N, P, and K/pot per application) was applied every other week. To protect
foliage from insect damage, plants were
30 sprayed every other week periodically
with 0.5 Ib per acre of a Bacillus
25 / thuringensis microbial insecticide to
< control the fern caterpillar (Callopistria
< Four pots replicationss) were used for
10 each treatment. Plants in the
experimental units were examined
5 weekly. All new leaves that had reached
Growth Stage 5 -6 ( young leaves, fully
o -- 30 45 60 75 expanded, but not yet hardened, leathery,
DAYS and dark green) were examined for
anthracnose infection. Infected leaves
Figure 2. Percent leaf area damaged under 1 = 30%, were rated for percent leaf area damaged
2 = 40%, 3 = 50%, 4 = 60%, 5 = 73%, and 6 = 80% b c t w
shade. by comparing them with a pictorial
damage key and the average percent leaf
area damaged (LAD) was calculated. The proportion of sampled leaves that were
infected was also calculated. Data were analyzed by a one-way ANOVA; means were
separated with Student-Newman-Keul's method (P 0.05).
Spore production by C. acutatum was studied by trapping spores at approximately 7-day
intervals within the canopy of diseased plants. Spores were trapped in plastic centrifuge
tubes buried with their opening placed about 2 cm above the soil level; two tubes were
placed in each pot. Spores of the pathogen are dispersed by water, so water provided by
natural rainfall or by rainfall simulated by overhead sprinkler irrigation was collected in the
tubes, and 1 ml samples were plated on a modified potato dextrose agar that was selective
for C. acutatum. After 5 or 6 days, the colonies of C. acutatum were counted and the
number of spores per ml in the collection tubes was calculated.
RESULTS AND DISCUSSION
At the end of the experiment, there were
significant differences in the levels of
disease damage between the treatments,5 /
but there were not significant differences in
the proportion of sampled leaves that were
infected (Figs. 1 and 2, Table 1). Disease
damage (percent leaf area damaged) was
significantly higher under 30 and 40 % //-E. ,
shade as compared to that measured in 50,
60, 73, and 80 % shade (Fig. 1, Table 1).
Although significant differences in the
proportion of leaves infected were not 15 30 45 6o 75
obtained at the end of this experiment, plots DAYS
obtained he end of this experiment, plts Figure 3. Number of new leaves produced per period
of the weekly sampling data (Fig. 1 and 2) under 1 = 30%, 2 = 40%, 3 = 50%, 4 = 60%, 5 = 73%
clearly indicate that both disease incidence and 6 = 80% shade.
(percent leaves infected) and damage
(LAD) tended to be greater under the higher light intensities provided by the 30 50%
shade fabric (Figs. 1 and 2, Table 1).
Leatherleaf fern plants grew poorly under the higher light intensities provided by 30 40%
shade fabric. Although there were no significant differences in the number of leaves
produced between the all levels of shade
employed (Fig. 3), but there were significant
differences in the size (height) of leaves
grown under 30 50% shade compared \
with leaves produced under 60 80% shade '
fabric. Leaf height was significantly reduced 25 -
in the 30 and 40 % shade treatments (Table
1); leaves produced in these treatments
were small, pale green and would not be \
marketable. Fern growth was best under 73
or 80 percent shade, but there were no o
apparent differences in anthracnose
incidence or damage under shade levels 2 2 3 40 45 50 55 6
between 50 and 80 percent (Table 1). Ays
Figure 4. Number of spores trapped under 1 = 30%,
2 = 40%, 3 = 50%, 4 = 60%, 5 = 73%, and 6 = 80%
Spore production as measured by periodic
spore trapping was quite variable throughout the experiment; this result is typical for C.
acutatum spore trapping data. There appeared to be no large or significant differences in
the number of spores produced among the different light intensity treatments (Fig. 4).
The results obtained in this experiment confirm grower observations that anthracnose
incidence and damage can be worse on plants growing in areas that receive relatively
high light levels as compared to the standard procedure of growing ferns in moderately
dense shade. However, the reasons for the observed effects could not be determined
from the results of this experiment. Increased levels of anthracnose observed under 30
and 40% shade are unlikely to have been caused by an increase in the initiation rate or
populations of new leaves, because no differences in leaf production were detected
among the light intensity levels used in this experiment. Results also indicated that there
were no large or significant increases in spore production in the shade treatments that
resulted in increased disease development.
Significant differences were found in the percent leaf area damaged on infected leaves.
This indicates that under the higher light intensities, conditions were apparently more
favorable for disease development. Although no significant differences were found in
the percent of leaves infected under the different light intensities used, the data clearly
indicate that the proportion of infected leaves were greater under the higher light
intensities. These results indicate that conditions for infection and disease development
were somehow enhanced under the 30 and 40 percent shade, but do not explain why
It is possible that different light intensities provided different microclimates or different
intervals of leaf wetness each day. However, increased light intensity would be
expected to reduce leaf wetness periods. It is more likely that the higher light intensities
could have produced significant physiological differences in the leaves; the smaller size
and pale-yellow color of these leaves supports this notion.
Table 1. Disease incidence, damage and some growth characteristics of leatherleaf fern
leaves grown under various levels of shade.
Percent Percent leaves1 Percent Leaf Number3 Average4
Shade infected area2 of new height
damaged leaves (cm)
30 66.42 12.25 a 17.71 18.04 e
40 69.02 9.50 a 15.00 23.60 abc
50 71.79 9.94 a 19.17 21.73 bcd
60 52.80 4.56 ab 18.13 26.16 a
73 40.15 2.12 b 20.00 24.10 ab
80 37.59 2.51 b 21.04 21.54 bcd
F= 2.24 4.451 0.069 7.569
P= 0.095 0.008 0.996 < 0.001
1ANOVA on arc sin transformations of values. Values followed by the same letter are
not significantly different P = 0.05.
2Average percent leaf area damaged on diseased leaves.
3Average number of new leaves (Growth Stage 5 and 6) produced during last
observation interval (2 weeks). An estimate of leaf production rate.
4Average length (height) of leaves produced.