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  • Author or Editor: Brian K. Hogendorp x
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Both laboratory and greenhouse experiments were conducted to determine if the fungicide, MilStop® (BioWorks, Victor, NY), which contains the active ingredient, potassium bicarbonate, has direct activity on the citrus mealybug, Planococcus citri Risso. Spray applications of four different rates (4.5, 5.9, 7.4, and 14.9 g·L–1) were applied to green coleus, Solenostemon scutellarioides (L.) Codd., plants infested with citrus mealybugs. In addition, experiments were conducted to assess both the direct and indirect effects of MilStop® on two natural enemies of the citrus mealybug: the parasitoid, Leptomastix dactylopii (Howard), and the coccinellid beetle, Cryptolaemus montrouzieri (Mulsant). MilStop® provided between 56% and 86% mortality of citrus mealybug; however, the highest rate (14.9 g·L–1) was phytotoxic to coleus plants. Percent mortality associated with the second highest rate (7.4 g·L–1) was 82%, which was comparable to acetamiprid (84%) applied at 0.05 g·L–1. For the natural enemies, MilStop® treatment rates of 1.5 and 3.5 g·L–1 resulted in 16% mortality, whereas the 5.5- and 9.0-g·L–1 rates resulted in 33% mortality of L. dactylopii adults. MilStop® treatment rates of 3.5, 5.5, 9.0, and 12.0 g·L–1 resulted in 30%, 60%, 40%, and 90% mortality, respectively, of C. montrouzieri adults. Therefore, depending on the application rate, this fungicide may inadvertently kill citrus mealybugs when used to control fungal plant pathogens. It should not disrupt biological control programs targeting citrus mealybug in greenhouses that involve releases of L. dactylopii when used at low application rates, whereas MilStop® applications should be properly timed when using C. montrouzieri.

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Although silicon is not an essential element, it is taken up by plants but is rarely quantified. Therefore, this study quantified the silicon concentration in 10 commonly grown horticultural plants including meadow sage (Salvia ×sylvestris), tickseed (Coreopsis verticillata), garden phlox (Phlox paniculata), New England aster (Symphyotrichum novae-angliae), Chinese astilbe (Astilbe chinensis), coral flower (Heuchera hybrid), garden zinnia (Zinnia elegans), French marigold (Tagetes patula), sweet basil (Basil spp.), and rosemary (Rosmarinus officinalis) using a plant alkaline fusion technique, which involved dry-ashing plant tissue samples and measuring color development with a spectrophotometer. Both zinnia and aster accumulated substantially more silicon from the municipal water source and growing medium (5365 and 4797 mg·kg−1 silicon, respectively) than the other plants evaluated, which had concentrations less than 2500 mg·kg−1 silicon. This study is just one of a few in which the silicon concentration in various horticultural plants has been quantified. Consequently, this may lead to better understanding those plants that will or will not benefit from applications of silicon-based fertilizers to promote cold-hardiness and/or plant resistance to fungal pathogens and insect pests.

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Mealybugs are major insect pests of greenhouses, interiorscapes, and conservatories feeding on a wide range of horticultural crops. However, mealybugs are difficult to regulate with insecticides as a result of the presence of a nearly impervious protective waxy covering, which means that alternative management strategies are required. As such, this study was designed to determine the value of applying silicon-based fertilizers such as potassium silicate to fiddleleaf fig, Ficus lyrata, plants as a means of alleviating outbreaks of the citrus mealybug, Planococcus citri. The study evaluated the effects of applying a commercially available silicon-based fertilizer product, ProTek® 0-0-3 The Silicon Solution, as a drench to the growing medium at different rates (0, 100, 400, 800, and 1600 ppm silicon). We determined the effect of the silicon-based fertilizer rate treatments on citrus mealybug life history parameters, including number of eggs laid by the female, body size (mm), and development time (days) from first instar to ovipositing adult female. In addition, we used a plant alkaline fusion technique to assess the concentration (mg·kg−1 or ppm) of silicon in the aboveground tissues (leaves and stems) of fiddleleaf fig plants at variable time intervals (days). This technique involves dry-ashing plant tissue in a muffle furnace followed by alkaline fusion and then colorimetric analysis. We found that the silicon-based fertilizer rate treatments did not negatively affect any of the citrus mealybug life history parameters measured. Citrus mealybug female egg load ranged from 132.3 to 159.2 and the development time (days) ranged from 66.9 to 68.7 d. The silicon concentrations present in the fiddleleaf fig plants on the final harvest date were between 4419.2 and 7241.7 mg·kg−1 silicon with fiddleleaf fig plants that received the 1600 ppm silicon-based fertilizer rate treatment having the highest silicon concentration. Moisture content was not significantly different among plants receiving the different silicon concentrations. Our results seem to suggest that fiddleleaf fig may actually be a silicon “rejector” and, as such, applications of silicon-based fertilizers are not beneficial to fiddleleaf fig plants because they do not accumulate sufficient quantities of silicon to impact citrus mealybugs.

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