Browsing Citrus Reports by Subjects
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Effect of Temperature and Moisture on Survival of Phytophthora in Citrus Grove SoilBefore replanting a citrus grove in Arizona, different preplant cultural activities may be performed, such as immediate replanting of the new citrus grove, allowing soil to lay fallow for various lengths of time, or planting the site to alfalfa for one or more years before the new citrus grove is established. A study was conducted to compare the effect of these different cultural preplant practices on the survival of Phytophthora in citrus grove soils. In June, 1998, and July, 1999, a total of 18 soil samples were collected within mature lemon groves. Each initial bulk sample was pretested, found to contain Phytophthora parasitica, then thoroughly mixed and partitioned into 1-liter plastic containers, which were subjected to different environmental and cultural conditions. The soil in each 1-liter container was tested for the presence of P. parasitica 1 and 3.5 to 4 months later. All soil samples then were placed in the greenhouse and a 6-month-old Citrus volkameriana seedling was planted in soil samples not containing plants. Three 1-liter sub-samples from each of ten 7-liter volumes of soil incubated outside for three months were also planted to citrus in the greenhouse. The soil containing plants in the greenhouse was watered as needed for 3 months, then again tested for the presence of Phytophthora. Irrigating soil infested with Phytophthora parasitica, whether it was planted to a host (citrus) of the pathogen, planted to a non-host (alfalfa) of the pathogen, or not planted at all, did not lower the pathogen to nondetectable levels. Phytophthora became and remained nondetectable only in the soil samples that were not irrigated and subjected to mean temperatures of 35 to 37° C (94 to 98° F). On the other hand, the pathogen was detectable in some soil samples subjected to dryness at lower mean temperatures of 26 to 30° C (79 to 86° F) after a citrus seedling subsequently was grown in the soil for 3 months. A dry summer fallow period following removal of a citrus grove (including as much root material as possible) was the only cultural practice among those tested that reduced the level of Phytophthora to nondetectable levels in all soil samples tested.
Evaluation of Fungicidal Management of Alternaria Rot on Citrus Fruit in 2000-2001 SeasonAlternaria fruit rot on Minneola tangelos and navel oranges can reach economically important levels in central Arizona. The objective of this study was to test the efficacy of a new fungicide in development, BAS 500, for disease management. A trial was conducted in a commercial Minneola tangelo grove with a history of Alternaria fruit rot. Within this grove, nine trees were sprayed monthly from August to December 2000 with BAS 500 at a rate of 0.25 lb active ingredient per acre. Another nine trees were not sprayed and served as controls. Disease severity was evaluated monthly from September 2000 to March 2001 by counting the number of infected fruit that had dropped from trees. No disease was evident on fruit from August through November, when fruit were green. By December the fruit had matured and turned color; additionally, the first fruit were detected with Alternaria fruit rot. In December and January there was little difference in the number of infected fruit on treated compared to nontreated trees. On the other hand, by February and March the cumulative number of infected fruit from trees treated with BAS 500 was 3.0 and 3.7 %, respectively, whereas the cumulative number of diseased fruit from nontreated trees during the same months was greater at 4.9 and 6.4 %, respectively. This study will be repeated next year with an adjusted spray schedule with the goal of increasing the level of disease control.
Impact of preplant soil treatments on survival of Phythophthora in citrus soilsSeveral different approaches are used with respect to land preparation prior to replanting citrus in Arizona. A study was initiated to examine the effect of cultural preplant practices on the survival of Phytophthora in citrus orchard soils. In June, 1998, a 2-gallon volume of soil was collected from eight different sites within a mature lemon planting on a sandy soil in Yuma or a lemon planting on a heavier soil in Mesa, AZ. Each initial sample was pre-tested, found to contain Phytophthora parasitica, then thoroughly mixed and distributed into six 1-qt plastic containers, which were subjected to different environmental and cultural conditions. The soil in each container was tested for the presence of P. parasitica 1, 4 and 9 months after initiation of the study. The preliminary results of this ongoing study are as follows. Detection of P. parasitica was lower in non-irrigated as compared to irrigated soil. P. parasitica was not detected in non-irrigated soil subjected to a mean temperature of 38°C (100° F) for 3 months. During the 9-month period of time, detection of P. parasitica in soil planted to alfalfa was not reduced compared to soil planted to citrus. Of the treatments examined, dry summer fallow may be the most effective method of reducing the population of P. parasitica to below detectable levels; however, these preliminary findings must be validated by additional planned tests.