INTRODUCTION
Cultural practices can be modified to improve control of Botrytis fruit rot in strawberry. These approaches focus on inoculum reduction through sanitation, or microclimate modification within the canopy to produce conditions less suitable for disease. One potential cultural practice is the use of wider within row plant spacings. Avoiding close plant spacings to promote air circulation and rapid drying within the fruiting canopy may be a way to reduce disease problems like Botrytis fruit rot that are favored by high humidity and extended periods of dew.
In
Florida, fruit harvested early in the season (November and December) receives a dramatically higher market price
than fruit harvested late in the season.
This requires growers to balance the benefits of higher planting densities
and potentially higher early yields with the risk of increased disease incidence. This study was undertaken to determine
the effects of within row plant spacing and cultivar on the incidence of Botrytis fruit rot marketable yield.
METHODS
Experiments were conducted during the 1997 to 1998 and 1998 to 1999 seasons at the University of Florida, Gulf Coast Research and Education Center at Dover. Three cultivars (Sweet Charlie, Camarosa, and Rosa Linda) and four within row plant spacings (9, 12, 15 and 18 inches) were evaluated each season. Canadian grown green-top bare root plants were transplanted into methyl bromide:chloropicrin (98:2) fumigated soil in plastic-mulched raised beds on 7 October 1997, and 16 October 1998. Plants were placed in two offset rows per bed, spaced 12 inches apart between rows and plots were 20 ft long. Weekly applications of captan (3.0 lbs a.i./acre) were applied by tractor at 100 gallons/acre to moderate the severity of Botrytis fruit rot.
Fruit were harvested twice weekly from 9 December 1997 to 30 March 1998 (33 harvests) and 30 November 1998 to 29 March 1999 (35 harvests). Each harvest was graded into marketable and unmarketable fruit. The marketable fruit were counted and weighed and unmarketable fruit sorted into number of fruit with Botrytis rot and other cull characteristics (i.e. other diseases, misshapen fruit, small fruit, etc.). For general comparisons, analyses of variance were performed for cumulative Botrytis fruit rot incidence and marketable yield for the early period (harvests from first 8 weeks), late period (all harvests after first eight weeks) and the whole season (all harvests). Mean comparisons were made by Fisher’s protected least significant difference (LSD) (P £ 0.05).
RESULTS
Severe epidemics of Botrytis fruit rot developed each season. Highly significant cultivar differences in Botrytis incidence were found for all three periods evaluated both seasons (Table 1). ‘Sweet Charlie’ was the most susceptible, ‘Camarosa’ was moderately susceptible and ‘Rosa Linda’ the least susceptible. The ranking of cultivar susceptibility was consistent during all three periods (early, late, and whole season) each season.
No significant spacing effect on cumulative Botrytis incidence was found during the early period during the 1997 to 1998 season (Table 1). Because of statistical interactions between cultivar and spacing, spacing effects were analyzed separately for each cultivar for the late and whole season periods. For the whole season, ‘Camarosa’ at the narrowest spacing had the highest Botrytis incidence, the intermediate spacings were intermediate, and the widest spacing had the lowest incidence. For the late period, only the narrowest spacing of ‘Camarosa’ had significantly higher Botrytis incidence than the wider spacings. No significant spacing effects were found for either ‘Rosa Linda’ or Sweet Charlie’ during the early, late or whole season periods.
In the 1998 to 1999 season, highly significant spacing differences on Botrytis incidence were found for the late and whole season periods (Table 1). Narrower plant spacings (9 and 12 inches) had significantly higher Botrytis incidence than wider spacings (15 and 18 inches).
Highly significant cultivar differences on total marketable yield occurred during all three periods evaluated each season (Table 1). During the 1997 to 1998 season ‘Camarosa’ had significantly higher marketable yield during the early period, and ‘Rosa Linda’ had higher yield during the late period. During the 1998 to 1999 season, the yield of ‘Rosa Linda’ was significant higher during all three periods. ‘Sweet Charlie’ had the lowest total cumulative marketable yield both seasons.
Marketable yields were significantly affected by plant spacing during the early and whole season periods of the1997 to 1998 season, and for all three periods during the 1998 to 1999 season. Both seasons, significantly higher marketable yield occurred at the narrower row spacings during the early period, and significantly lower marketable yield at the widest spacing (18 inches) for the whole season period.
CONCLUSIONS
This study revealed that wider within row plant spacing reduced the incidence of Botrytis fruit rot. Different levels of susceptibility to Botrytis fruit rot were found for the three cultivars evaluated in this study, and the differences were consistent throughout both seasons. Plant spacing also significantly affected marketable yield. Narrower spacings produced higher early period and whole season yields than wider spacings.
Many different factors may be responsible for the significant differences that plant spacing had on Botrytis fruit rot incidence. Increased plant density could reduce the effectiveness of fungicide applications by preventing good plant coverage. The increase in Botrytis incidence related to decreased plant spacing could have been related to reduced fungicide coverage. Increased plant density could also affect the harvesting of fruit late in the season when plants became larger and harvesting more difficult. In narrower plant spacings, more fruit are concealed and unharvested fruit may contribute to higher inoculum levels, and thus more disease.
No significant spacing differences on Botrytis fruit rot incidence were detected during the early period of either season. This was expected since strawberry plants in annual production systems are small early in the season. Spacing effects are most likely to occur in February or March when the canopy closes and the plants become large enough to influence the development of Botrytis fruit rot epidemics.
A highly significant cultivar effect on Botrytis fruit rot incidence was observed both seasons. ‘Sweet Charlie’ was the most susceptible cultivar evaluated in this study and is grown extensively in Florida (40 to 50% of acreage). This cultivar is preferred by growers due to its ability to produce high quality, good flavored, early season fruit, and it’s high level of resistance to anthracnose fruit rot (Colletotrichum acutatum). Growers frequently suffer high losses to Botrytis fruit rot when using this cultivar, and rely on regular fungicide programs to minimize losses. The other main cultivar grown in Florida is Camarosa (30 to 40% of acreage). ‘Camarosa’ is less susceptible to Botrytis than ‘Sweet Charlie’, but it does not normally produce high early season yields, and is extremely susceptible to anthracnose fruit rot. ‘Rosa Linda’ was the cultivar least susceptible to Botrytis fruit rot. This cultivar is also less susceptibility to anthracnose fruit rot than ‘Camarosa’ although it is not as resistant as ‘Sweet Charlie’. ’Rosa Linda’ appears to be a cultivar that should be considered by growers hoping to minimize fruit rot problems and maximize marketable yields.
To increase profits growers try to maximize marketable yields for both the high value early period and for the whole season. For winter strawberry producers, prices during the early part of the season are often 2 to 5 times the price of fruit harvested the rest of the season. According to this study, early yield can be significantly increased by using narrower plant spacings, but at the risk of increased Botrytis fruit rot incidence later in the season. Either increases in total production or increased early production must offset the increased plant costs from using narrower spacings and the increased disease losses. When narrow plant spacing is used to increase early yield, growers should plant cultivars like Camarosa or Rosa Linda that are less susceptible to Botrytis fruit rot.