No abstract

An improved knowledge of effects of density of plants on yield of watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] would help efforts to determine optimal planting density and to anticipate the economic impact of factors that reduce density. We conducted a series of experiments to determine plant density‐dependent rates of change of marketable yield, fruit biomass, and marketable fraction in watermelon cultivar Sugar Baby. In single‐row plots, at least 3.7 m apart, density varied from 0.4 to 4.1 plants m2 (1000‐9000 plants ha−1). Marketable yield per unit area increased at linear rates of 0.5 to 1.1 Mg ha−1 per thousand plants ha−1 because fruit biomass increased at linear rates of 1.1 to 3.2 Mg ha−1 per thousand plants ha−1. The linear effect of plant density explained more than 90% of the increase in fruit biomass per unit area in most experiments. Density did not affect the fraction of fruit biomass that was of marketable quality. The linear rate of change in the marketable fraction did not exceed 3% per 1000 plants ha−1 on average in any experiment. Per plant, marketable yield and fruit biomass, respectively, decreased at curvilinear rates of 0.8 to 8.6 and 1.4 to 10.8 (kg plant−1 per thousand plants ha−1) (plants ha−1)2. These decreases were consistent with a constraint due to intraspecific competition. Our results support the hypothesis that efficiency of commercial production of watermelon could be increased by increasing planting densities.

In controlled environment studies, the influence of temperature and wetness duration on infection of strawberry leaves by Mycosphaerella fragariae was quantified by inoculating plants with a conidial suspension and incubating them at various combinations of temperature (5 to 35 degrees C) and leaf wetness duration (0 to 96 h). Infection was expressed as the number of lesions per square centimeter of leaf surface and relative infection was used to develop an infection model. Younger leaves were more susceptible to infection. Regardless of temperature and duration of leaf wetness, only few lesions developed on the oldest (19 to 21 days old) and intermediate leaves (12 to 15 days old), respectively (maximum of 1.7 and 2.3 lesions per cm(2)) as compared to the youngest leaves (5 to 7 days old; maximum of 12.6 lesions per cm(2)). On the youngest leaves, lesions developed at all temperatures except at 35 degrees C, and the number of lesions, for all leaf wetness durations, increased gradually from 5 to 25 degrees C and decreased sharply from 25 to 30 degrees C. For temperatures of 15 and 20 degrees C, the number of lesions increased gradually when leaf wetness duration increased from 12 to 96 h. At 25 degrees C, the number of lesions increased with increasing leaf wetness from 12 to 48 h and then at a higher rate from 48 to 96 h. The optimal temperature for infection was 25 degrees C. For most temperatures, a minimum of 12 h of leaf wetness was necessary for infection (more than 1 lesion per cm(2)). Relative infection was modeled as a function of both temperature and wetness duration using a modified version of the Weibull equation (R (2) = 0.98). The resulting equations provided a precise description of the response of M. fragariae to temperature. The model was sufficiently flexible to account for most characteristics of the response of M. fragariae to wetness duration. The model was used to construct a risk chart that can be used to estimate the potential risk for infection based on observed or forecasted temperature and leaf wetness duration.

The relationship between dose for each of four biorational insecticides (pyrethrins, neem extract, capsiacin extract, insecticidal soap) and mortality of the green peach aphid (Myzus persicae) was determined using a laboratory bioassay. These insecticides were toxic to aphids and paired mixtures of the insecticides provided synergistic activity as measured by aphid mortality under the laboratory bioassay conditions. Capsiacin extracts were found to provide low levels of mortality alone but acted synergistically in mixtures with the other insecticides and provided higher than expected levels of mortality. Activity as determined in the laboratory for each insecticide was not evident under field-use conditions in five separate experiments. Under field conditions and using common application methods, these insecticides did not provide significant levels of control of aphids.

Infection of stem bases of winter wheat by Fusarium graminearum was directly related to the incidence of infection of seed at planting. The efficiency of transmission of the pathogen from seed to stem ranged from 55 to 94% over four sampling dates in two trials. The incidence of infection of stem bases did not increase between autumn and the following summer, indicating that all transmission occurred during the autumn. Per cent germination of seed and stand density decreased as the incidence of infected seed increased.

Quantitative analyses of the continuous response of components of the biomass of fruits of watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] to variation in plant density could provide insight into the mechanisms underlying effects of plant density on marketable yield. Per unit area, the linear response of fruit biomass to plant density recently has been shown to explain the linear response of marketable yield. In the current study we quantify plant density‐dependent variation in the size, density (no. per unit area), and frequency (no. per plant) of watermelon fruits. In single‐row plots, at least 3.7 m apart, plant density varied from 0.4 to 4.1 plants m2 (1000–9000 plants ha−1). In each experiment, the linear effect of plant density explained more than 80% of the variation in fruit density. Fruit density increased at linear rates of 0.6 to 1.1 thousand fruits ha−1 per thousand plants ha−1. The plant density‐dependent response of the size of fruits varied considerably among experiments but the frequency of fruits responded consistently. In four experiments, there was no evidence of an effect of plant density on fruit size but in three experiments, fruit size decreased at a curvilinear rate of approximately 2.0 (kg−1 fruit−1 per thousand plants−1 ha−1)(plants ha−1)2. Frequency of fruits decreased with plant density at curvilinear rates of 0.8 to 2.8 (fruits plan−1 per thousand plants ha−1)(plants−1 ha−1)2. The response of size of fruits and frequency of fruits, respectively, probably measured an environment‐dependent and an environment‐independent effect of intraspecific competition.

The effects of temperature and duration of wetness (relative humidity >/=95%) on infection of three peanut cultivars by Cercospora arachidicola were determined under controlled conditions. Plants of the Spanish cv. Spanco and the runner cvs. Florunner and Okrun were exposed to constant temperatures of 18 to 30 degrees C during 12-h periods of wetness each day that totaled 12 to 84 h following inoculation of leaves with conidia. Severity of disease, measured by either lesion density (number per leaf) or lesion size (diameter), was greatest for 'Spanco', intermediate for 'Florunner', and lowest for 'Okrun' in each of two experiments. Lesion density was evaluated further because it was an indicator of both the occurrence and degree of infection. Nonlinear regression analysis was employed to evaluate the combined effects of temperature (T) and wetness duration (W) on lesion density (Y). In the regression model, the Weibull function characterized the monotonic increase of Y with respect to W, while a hyperbolic function characterized the unimodal response of Y with respect to T. Parameters for the intrinsic rate of change with respect to W (b), the intrinsic rate of change with respect to T (f), the optimal value of T (g), and the upper limit (e) when T is optimum (T = g) were estimated for each cultivar and experiment. The effect of cultivar was characterized primarily by differences in the upper limit parameter e. In each experiment, e was greatest for 'Spanco', intermediate for 'Florunner', and least for 'Okrun'. The effect of cultivar on b followed a pattern similar to that for e in experiment 1, but not in experiment 2. Differences among cultivars for estimates of f and g were small and inconsistent. Estimates for g were precise for each cultivar and experiment and fell within the range of 22.3 to 23.2 degrees C. Cultivar responses to T and W were further evaluated using data pooled over the two experiments. Parameter e was estimated for each cultivar, but common values of b, f, and g were estimated. At e = 22.8 degrees C, lesion density approached an upper limit of 96, 17, and 6 lesions per leaf for the cvs. Spanco, Florunner, and Okrun, respectively. These fitted values approximated the observed values of 86, 25, and 9 lesions per leaf for the respective cultivars. Cultivars varied in their response to W at a given T. At 22.8 degrees C, one lesion per leaf was expected following 26, 30, and 36 h of wetness for 'Spanco', 'Florunner', and 'Okrun', respectively. If temperature was increased to 28 degrees C, one lesion per leaf was expected following 36, 44, and 54 h of wetness for the respective cultivars.

Adult squash bugs, Anasa tristis (De Geer) (Heteroptera: Coreidae), were confined on watermelon plants at differing phenological stages and at densities of zero to four per plant in one trial and zero to 32 per plant in three additional trials. Squash bugs were allowed to feed on the plants until plants died or fruit matured. Plant foliage, roots, and fruit were harvested and weighed to determine effects on growth and productivity. Growth and fruit production was regressed on number of squash bugs and results indicated that an increasing density of squash bugs feeding on vining and flowering stage plants resulted in significant reductions in plant growth and fruit yield. When plants were infested at the fruit set stage of growth, there was either less effect or no effect on plant growth and fruit production. Plant mortality increased as the density of squash bugs increased regardless of the stage of growth when plants were infested with squash bugs.