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.