C. K. Chandler

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.

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.

Resistance of Strawberry Cultivars to Crown Rot Caused by Colletotrichum gloeosporioides Isolates from Florida Is Nonspecific

Isolates of Colletotrichum gloeosporioides from strawberry (Fragaria × ananassa) and native grape were tested for virulence on strawberry cultivars in field experiments for three seasons. Isolate aggressiveness and cultivar resistance were determined by the proportion of plants killed at a defined time. Each year, four to six isolates were inoculated on four to seven different cultivars, with a subset of isolates and cultivars evaluated again the next season. On the dates that disease was evaluated, incidence ranged from 10 to 84% for individual cultivars. Cultivar and isolate effects were significant in all three seasons, but there was no significant cultivar by isolate interaction in any season. Thus, resistance to C. gloeosporioides appears to be nonspecific. In the third season, one isolate of Colletotrichum fragariae from strawberry and one from oak were included. There was no significant cultivar by isolate interaction detected for this species, although there were significant differences among cultivars and isolates. When the resistance of cultivars to both species was compared, the rankings of cultivars were similar, but a modest cultivar by species interaction was evident. The cultivar Treasure was more resistant to crown rot caused by either species than any other cultivar tested.

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.

Reduced Sampling Frequency for Evaluating Fungicide Efficacy on Botrytis Fruit Rot of Strawberry

Evaluating fungicide efficacy in annual strawberry production systems can be labor intensive due to continuous harvesting over a relatively long season. The effect of reduced harvest number on the accuracy of least significant difference (LSD) separations for Botrytis fruit rot (Botrytis cinerea) incidence and marketable yield in fungicide efficacy studies was evaluated over three seasons. Fruit were harvested and graded twice a week for a total of 23 to 32 harvests each season. Data from each season were divided into different sample sets. Data from three different harvest periods (early, late, and whole season) and different harvesting frequencies (twice weekly, once weekly, every second, third, or fourth week) were compared with the complete data set (twice weekly for the whole season). Spearmans rank correlation and Pearsons product moment correlation coefficients were used to evaluate the correlation of the complete data sets with data sets from other sampling plans. Harvesting once a week for either the late- or whole-season periods accurately estimated LSD groupings for Botrytis fruit rot incidence among fungicide treatments. The precision of marketable yield estimates using once-a-week harvesting for the late or whole-season periods were relatively lower than for the incidence of Botrytis.