Phytophthora root rot (PRR), caused by Phytophthora cinnamomi, is the most destructive disease of avocado worldwide. In the United States, mefenoxam and phosphonate products are currently the only registered fungicides for managing avocado PRR. Four new Oomycota-specific and two registered fungicides, all with different modes of action, were evaluated. Seventy-one isolates of P. cinnamomi from avocado in California, most of them collected between 2009 to 2017, were tested for their in vitro sensitivity to the six fungicides. Baseline sensitivity ranges and mean values (in parentheses) of effective concentrations to inhibit mycelial growth by 50% (EC50) for the new fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin were 0.017 to 0.069 μg/ml (0.035), 0.046 to 0.330 μg/ml (0.133), 0.003 to 0.011 μg/ml (0.005), and 0.0002 to 0.0007 μg/ml (0.0004), respectively. In comparison, the EC50 value range (mean) was 0.023 to 0.138 μg/ml (0.061) for mefenoxam and 12.9 to 361.2 μg/ml (81.5) for potassium phosphite. Greenhouse soil inoculation trials with 8-month-old Zutano seedlings and 10-month-old Dusa and PS.54 clonal rootstocks were conducted to assess the efficacy of these fungicides for managing PRR. Mefenoxam and potassium phosphite were effective treatments; however, oxathiapiprolin, fluopicolide, and mandipropamid were more effective. Ethaboxam was effective in reducing PRR on the rootstocks evaluated. Oxathiapiprolin reduced PRR incidence and pathogen population size in the soil by >90%, and plant shoot growth and root dry weight were significantly increased compared with the control; thus, oxathiapiprolin was one of the best treatments overall. The high activity and performance of these new fungicides supports their registrations on avocado for use in rotation and mixture programs, including with previously registered compounds, to reduce the risk of development and spread of resistance in pathogen populations.
Phytophthora root rot (PRR) of avocado (Persea americana), caused by Phytophthora cinnamomi, is the most serious disease of avocado worldwide. Previous studies have determined that this pathogen exhibits a primarily clonal reproductive mode but no population level studies have been conducted in the avocado-growing regions of California. Therefore, we used amplified fragment length polymorphism based on 22 polymorphic loci and mating type to investigate pathogen diversity from 138 isolates collected in 2009 to 2010 from 15 groves from the Northern and Southern avocado-growing regions. Additional isolates collected from avocado from 1966 to 2007 as well as isolates from other countries and hosts were also used for comparative purposes. Two distinct clades of A2 mating-type isolates from avocado were found based on neighbor joining analysis; one clade contained both newer and older collections from Northern and Southern California, whereas the other clade only contained isolates collected in 2009 and 2010 from Southern California. A third clade was also found that only contained A1 isolates from various hosts. Within the California population, a total of 16 genotypes were found with only one to four genotypes identified from any one location. The results indicate significant population structure in the California avocado P. cinnamomi population, low genotypic diversity consistent with asexual reproduction, potential evidence for the movement of clonal genotypes between the two growing regions, and a potential introduction of a new clonal lineage into Southern California.
Phytophthora cinnamomi, the causal agent of Phytophthora Root Rot (PRR) of avocado, is the most serious disease of avocado worldwide. The development of tolerant rootstocks to control PRR has proven to be an effective means to control the disease. However, using traditional breeding approaches can take over a decade to produce results and there has been a limited use to integrate molecular data into breeding efforts. Therefore, the objective of this work was to use amplified fragment length polymorphism (AFLP) to genetically characterize greenhouse selected PRR tolerant rootstocks, some that had been field-tested and others that had not. Sixty-one polymorphic markers were identified among the 83 cultivars/selections tested. Based on UPGMA analysis, extensive genotypic diversity was found with the closest genotypes differing by six markers. No obvious trends were observed based on the cluster analysis with regards to known tolerant and susceptible rootstocks but rootstocks that have been field-tested and determined to be very tolerant rootstocks such as Latas, Dusa, PP14 (Uzi), PP4 (Zentmyer), and PP24 (Steddom) were distributed across the tree. This suggests that the same mechanisms for resistance may not be shared among these various selections. Therefore, these best-performing and most genetically diverse rootstocks will be combined in future breeding efforts to potentially pyramid diverse sources of resistance into future selections.
The role of honeybees (Apis mellifera) in the
pollination of buckwheat cv. Manor was studied in a commercial planting at
Smeaton, Victoria. Honeybees comprised 80% of all insect visitors to
this crop. Other insects included ladybirds
(Coccinella transversalis and
C. undecimpunctata), hoverflies
(Meangyna viridiceps), drone flies
(Eristalis sp.), blowflies (Calliphoridae), cabbage
white butterflies (Pieris rapae), small bush flies and
native bees. The activity of honeybees and other insects increased seed
production from 91.5 g/plot (plots closed to insects) to 180.4 g/plot
(plots open to insects).
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