J. C. Mertely

Comparison of Epidemics of Botrytis Fruit Rot and Powdery Mildew of Strawberry in Large Plastic Tunnel and Field Production Systems

Epidemics of Botrytis fruit rot (Botrytis cinerea) and powdery mildew (Sphaerotheca macularis f. sp. fragariae) in annual strawberry were compared in large plastic tunnel and field production systems during the 1998-99 and 1999-2000 seasons. Treatments were factorial combinations of two main plots (field and tunnel) and four subplots, including combinations of two cultivars (Camarosa and Sweet Charlie) and two captan schedules arranged in a split-plot design with three replications. The mean incidence of Botrytis fruit rot was 88 to 94% lower in tunnels than in the field. The incidence of Botrytis fruit rot for the untreated control in tunnels was less than 2%, which was 89% lower than that of the 7-day captan schedule in the field. This indicates that Botrytis fruit rot can be controlled effectively without fungicides in a tunnel cultural system. Powdery mildew was severe on susceptible cultivar Camarosa in tunnels. Early season yields of cultivar Sweet Charlie were significantly higher in tunnels than in the field. Shorter periods of leaf wetness and higher temperatures in tunnels may have contributed to a lower incidence of Botrytis fruit rot and a higher incidence of powdery mildew on fruit in tunnels compared with open field plots.

Timing of Fungicide Applications for Botrytis cinerea Based on Development Stage of Strawberry Flowers and Fruit

During the 1999-2000 and 2000-2001 growing seasons, field experiments were conducted to identify the developmental stage(s) of strawberry flowers and fruit that requires fungicide applications to control Botrytis fruit rot. Fenhexamid, a protectant fungicide, was applied to individual newly opened flowers or fruit of cultivar Sweet Charlie at defined intervals after anthesis. In 1999-2000, a single application of fenhexamid at anthesis controlled Botrytis fruit rot as well as multiple weekly applications beginning at anthesis. During both seasons, disease control deteriorated as applications were delayed 7 and 14 days after anthesis. This trend was described by linear regression equations relating the time of application to Botrytis fruit rot incidence. Additional treatments tested the effects of emasculation and petal removal 3 to 7 days after anthesis. Emasculation significantly reduced disease incidence in 2000-2001. Petal removal produced modest but significant reductions in 1999-2000, but not in 2000-2001. These results demonstrate that strawberry flowers are more susceptible to Botrytis cinerea than green fruit, and suggest that stamens are the principal infection court. Fungicide applications should focus on peak bloom periods to minimize fungicide use and optimize control of preharvest Botrytis fruit rot. During these periods, applications should be made at close intervals (≤7 days) to minimize losses to Botrytis.

Effects of Plant Spacing and Cultivar on Incidence of Botrytis Fruit Rot in Annual Strawberry

The effects of within-row plant spacing and cultivar on the incidence of Botrytis fruit rot (Botrytis cinerea) and marketable yield of annual strawberry were evaluated during the 1997-98 and 1998-99 seasons. Three cultivars (Camarosa, Rosa Linda, and Sweet Charlie) and four plant spacings (23, 30, 38, and 46 cm) were evaluated. Marketable yield and the incidence of Botrytis fruit rot were determined twice weekly. Cultivar and spacing effects were analyzed for three periods each season (early, late, and whole season). In 1997-98, spacing effects were observed on weekly incidence of Botrytis rot for the late period (P = 0.0925) and on cumulative incidence for the whole season period (P = 0.0795). Further analysis of the late and whole season periods revealed a spacing effect for Camarosa (P = 0.0102). Spacing also had a dramatic effect on cumulative and weekly Botrytis incidence for the late and whole season periods during the 1998-99 season (P ≤ 0.0014), when narrower spacings had higher incidence of Botrytis than wider spacings. Marketable yields were higher at narrower spacings during the early period for both seasons. Whole season marketable yields were also higher at the narrower spacings despite higher incidence of Botrytis. There were significant differences in susceptibility among cultivars.

Development of a reduced use fungicide program for control of botrytis fruit rot on annual winter strawberry

Crop phenology and epidemiological information were used to design a reduced use fungicide program for control of Botrytis fruit rot in winter annual strawberry. Fungicide spray programs during early and late periods of the season using high and low rates of captan were evaluated with or without second peak bloom applications of fenhexamid during the 1999-2000 and 2000-2001 seasons. During the early harvest period, low rates of captan were as effective as high rates for controlling Botrytis fruit rot and maintaining yield. Late in the season, treatments with fenhexamid over the peak bloom period significantly improved control of Botrytis fruit rot and increased marketable yield. Application of both captan and fenhexamid during the second peak bloom did not reduce Botrytis fruit rot incidence or improve yield compared with fenhexamid alone during this time period. Late season applications of captan may be reduced or eliminated when bloom applications of fenhexamid are being applied without affecting Botrytis fruit rot control. The study generated new recommendations for use of low-rate applications of captan during the early season and applications of fenhexamid during the second peak bloom period for winter annual strawberry production in Florida.

Management of Botrytis Fruit Rot in Annual Winter Strawberry Using Captan, Thiram, and Iprodione

The management of Botrytis fruit rot on annual strawberry by fungicides was evaluated in Florida during the 1995-96, 1996-97, and 1997-98 seasons. Weekly applications of captan or thiram, bloom applications of iprodione applied twice during each of two peak flowering periods, and weekly applications of captan combined with iprodione bloom applications were evaluated. Significant treatment effects (P ≤ 0.05) on the incidence of Botrytis fruit rot were detected for the early, late, and whole-season periods each season. Weekly applications of captan or thiram controlled Botrytis fruit rot, reducing disease incidence by more than 41% compared to the untreated control. These treatments also affected marketable yield during two seasons, with a 42 to 127% increase in yield compared to the control. Weekly fungicide applications did not reduce the incidence of Botrytis fruit rot until at least the fourth week of harvest, 9 to 10 weeks after applications began. Bloom applications of iprodione alone reduced the incidence of Botrytis fruit rot during the second peak bloom period, and the reductions in incidence occurred 1 to 3 weeks after the start of bloom applications. This suggests that iprodione applications control infections at flowering or early stages of fruit development. However, early-season bloom applications did not reduce the incidence of Botrytis fruit rot. The control of Botrytis fruit rot by weekly captan applications was not improved by the addition of iprodione bloom applications. These data suggest that early-season fungicide applications for the control of Botrytis fruit rot in annual winter strawberry are of limited efficacy, and that bloom applications of Botryticides such as iprodione should be focused on the second peak bloom period.

Comparison of sanitation and fungicides for management of botrytis fruit rot of strawberry.

To determine the effects of sanitation on yield and incidence of Botrytis fruit rot (Botrytis cinerea) in annual strawberry, replicated experiments were conducted during the 1995-96, 1996-97, and 1998-99 seasons. Leaf sanitation (removal of senescent and necrotic leaves) and fruit sanitation (removal of unmarketable fruit from alleys between beds) were compared to a standard fungicide control program (weekly applications of captan plus four bloom applications of iprodione) and combined sanitation and fungicide treatments. Leaf sanitation reduced Botrytis fruit rot incidence from 12.6 to 8.2% over the entire 1996-97 season, and from 17.6 to 11.8% during the latter half of the 1998-99 season, compared to untreated controls. However, sanitation did not increase marketable yield. Supplementing fungicides with leaf sanitation or leaf and fruit sanitation did not improve disease control and frequently reduced yield. Fruit sanitation had no significant effect on Botrytis incidence or yield. Losses to Botrytis fruit rot in the sanitation treatments were significantly higher (P ≤ 0.05) than in the fungicide treatments each season; marketable yields were significantly lower in 1996-97 and 1998-99. Under Florida conditions, fungicides control Botrytis fruit more effectively and economically than does sanitation.

First Report of Macrophomina phaseolina Causing a Crown Rot of Strawberry in Florida

Strawberry (Fragaria × ananassa Duchesne) is produced as an annual winter crop in raised, plastic-mulched beds on 2,800 ha in west central Florida. In December 2001, a grower submitted collapsed and dying strawberry plants from a commercial field to the University of Florida in Dover. The cut crowns of affected plants revealed dark brown necrotic areas on the margins and along the woody vascular ring. Macrophomina phaseolina was isolated from pieces of infected tissue cut aseptically from the crowns and placed on a medium containing 12 g of Difco potato dextrose broth, 17 g of Bacto agar, 250 mg of ampicillin, and 100 mg of streptomycin sulfate per liter of water. The fungus produced numerous, dark, oblong sclerotia in the isolation medium after 4 to 5 days incubation at 24°C under constant fluorescent lighting. In 10-day-old cultures, sclerotia ranged in size from 55 to 190 μm long by 50 to 135 μm wide (average 105 × 74 μm). Ostiolate pycnidia bearing relatively large, broadly ellipsoidal, hyaline conidia occasionally developed on the host tissue after 8 to 10 days of incubation (2). During the 2003-2004 season, M. phaseolina was isolated from dying strawberry plants taken from the original field and two additional farms. Affected plants were often found along field margins or other areas inadequately fumigated with methyl bromide. Two single-spore isolates from different fields were tested for pathogenicity on nursery runner plants (cv. Strawberry Festival) grown for 4 weeks in the greenhouse on artificial potting soil. The fungal isolates were grown on corn meal agar at 24°C for 4 days and allowed to colonize sterile wooden toothpicks placed on the medium for an additional 5 days. Prior to use, the toothpicks were sterilized by autoclaving twice in deionized water and a third time in V8 juice. Six plants were inoculated with each isolate by inserting a colonized toothpick into each crown. Sterile, V8-infused toothpicks were inserted into the crowns of corresponding control plants. The plants were incubated in a greenhouse in a randomized complete block design with two replicates of three plants each. After 3 days, 33 to 100% of the inoculated plants developed wilting in one or more leaves. All inoculated plants collapsed or died within 2 weeks of inoculation, while the control plants remained healthy during the observation period. The pathogen was readily reisolated from inoculated plants. Charcoal rot disease caused by M. phaseolina has been reported on strawberry in France, India, and Illinois (2,3). To our knowledge, this is the first report from Florida. M. phaseolina may be an emerging threat as the Florida strawberry industry transitions from methyl bromide to other fumigants in 2005. References: (1) J. Maas. Macrophomina leaf blight and dry crown rot. Page 26 in: Compendium of Strawberry Diseases. 2nd ed. J. L. Maas, ed. The American Phytopathological Society, St. Paul, MN, 1998. (2) G. S. Smith and T. D. Wyllie. Charcoal rot. Pages 29-31 in: Compendium of Soybean Diseases. G. L. Hartman et al., eds. 4th ed. The American Phytopathological Society, St. Paul, MN. 1999. (3) B. Tweedy et al. Plant Dis. Rep. 42:107, 1958.

Curative and Protectant Activity of Fungicides for Control of Crown Rot of Strawberry Caused by Colletotrichum gloeosporioides

The ability of fungicides to control Colletotrichum crown rot of strawberry caused by C. gloeosporioides was examined over three seasons. A single application of each fungicide was made 2 days before inoculation (2 DBI) or 1 day after inoculation (1 DAI) with conidial suspensions of C. gloeosporioides. The proportion of plants collapsed on one date at the end of each season was evaluated. In a combined analysis, there was a significant fungicide treatment-season interaction (P = 0.004). Percent mortality was 64% over 3 years in control plots that were inoculated with C. gloeosporioides but not treated with fungicide. Captan applied 2 DBI consistently reduced plant mortality (mean mortality = 17%). However, it was not as effective when applied 1 DAI (mean mortality = 46%). Azoxystrobin, pyraclostrobin, and thiophanate-methyl all reduced plant mortality relative to the control if applied 2 DBI (mean mortality = 46% for azoxystrobin, 37% for pyraclostrobin, and 41% for thiophanate-methyl) or 1 DAI (mean mortality = 29% for azoxystrobin, 27% for pyraclostrobin, and 32% for thiophanate-methyl). Results indicated that these fungicides were more effective when applied 1 DAI; however, lower plant mortality was not always observed with postinoculation applications. Cyprodinil + fludioxonil reduced mortality relative to the control, but there was no consistent evidence that it was more effective when applied at 2 DBI (mean mortality = 39%) than when applied 1 DAI (mean mortality = 40%). Similarly, mortality in plots treated with thiram 2 DBI (mean mortality = 30%) or 1 DAI (mean mortality = 32%) was not different. Potassium phosphite did not affect mortality, regardless of the timing of application (2 DBI mean mortality = 61%, 1 DAI mean mortality = 67%). The results indicated that an effective strategy for controlling Colletotrichum crown rot caused by C. gloeosporioides should be based on weekly applications of captan throughout the growing season. Azoxystrobin, pyraclostrobin, or thiophanate-methyl applications should be applied when weather conditions are highly favorable for disease development and the activity of contact fungicides such as captan or thiram might be compromised.

Colletotrichum fragariae Is a Pathogen on Hosts Other Than Strawberry

Evidence that Colletotrichum fragariae causes disease on hosts other than strawberry is limited. In the fall of 2006, fungal isolates from silver date palm with leaf spot symptoms and from cyclamen with leaf spot and stem rot symptoms were identified as C. fragariae. After confirming the pathogenicity of the isolates on their host of origin, a representative isolate from each host was compared to C. fragariae and to C. gloeosporioides/Glomerella cingulata isolates from strawberry. Date palm and cyclamen isolates bore conidia on setae, and conidia were tapered and in the size range of C. fragariae reference isolates. Sequence data from the combined internal transcribed spacer (ITS) regions 1 and 2 and the gene for the 5.8 ribosomal RNA from the cyclamen and date palm isolates matched the sequence for C. fragariae reference isolates. Based on these characteristics, it was concluded that the C. fragariae species designation was correct for both isolates. However, the date palm isolate was a weak pathogen on strawberry compared with other isolates and had a distinct AT-rich DNA banding pattern. The ability of the cyclamen isolate to cause crown rot on strawberry was comparable with the strawberry reference isolates, and the AT-rich DNA banding pattern of the cyclamen isolate was identical to the C. fragariae isolates from strawberry. The results indicate that C. fragariae is a pathogen on hosts other than strawberry and that there is more diversity among C. fragariae isolates than previously reported.

Uniformity of Strawberry Yield and Incidence of Botrytis Fruit Rot in an Annual Production System

A plant yield and disease incidence uniformity trial was conducted to provide information concerning the efficiency and precision of field trials used to evaluate Botrytis fruit rot control methods on strawberry. Fruit yield and Botrytis fruit rot incidence were recorded for individual strawberry plants of cultivars Sweet Charlie and Camarosa grown in an annual production system over two growing seasons. A nested analysis of variance model was used to measure plot edge effects and to obtain variance components to describe the relationship between plot size and plot variance. Mean seasonal yield for Sweet Charlie was 599 g/plant and for Camarosa 972 g/plant. Mean seasonal fruit rot incidence was 10.3% for Sweet Charlie and 3.0% for Camarosa. Plants growing on the edge of plots next to aisles had higher yields (637 versus 577 g/plant for Sweet Charlie and 1047 versus 923 g/plant for Camarosa), but there was no edge effect for disease incidence. Smiths equation was used to determine the relationship between plot size and plot variance within mulched beds. Smiths index was relatively high for yield (0.92 and 0.95) and Botrytis fruit rot incidence (0.91 and 0.69) for Sweet Charlie and Camarosa, respectively. This suggests a relatively uniform distribution of seasonal yield and disease incidence among plants within beds. Using plot variances estimated from Smiths equation, the power of hypothesis tests to discriminate hypothetical treatment effects of different magnitudes was examined for each cultivar. Power analysis suggests that treatment effects ranging from 11.3 to 21.3% of the mean seasonal yield observed for Sweet Charlie and 8.6 to 16.5% of the mean seasonal yield observed for Camarosa can be detected with a power of 0.80 using four replicates of each treatment group and plot sizes ranging from 32 to eight plants/plot. For Botrytis fruit rot incidence, a power of 0.80 is achieved with treatment differences ranging from 24.9 to 44.1% of mean seasonal Botrytis fruit rot incidence for Sweet Charlie and from 44.1 to 64.9% of the mean observed for Camarosa using plot sizes ranging from 32 to 8 plants/plot. Before planting, the crown diameter and leaf number of transplants were recorded. There was no consistent significant correlation between any of these growth traits and seasonal yields or Botrytis fruit rot incidence. However, there was a consistent positive correlation for both cultivars between yield and crown diameter and also yield and leaf number during the months of December and January in the early part of the season (0.1513 ≤ r ≤ 0.314 for crown diameter and 0.106 ≤ r ≤ 0.264 for leaf number).