Koon-Hui Wang

Transcription factor Amr1 induces melanin biosynthesis and suppresses virulence in Alternaria brassicicola.

Alternaria brassicicola is a successful saprophyte and necrotrophic plant pathogen. Several A. brassicicola genes have been characterized as affecting pathogenesis of Brassica species. To study regulatory mechanisms of pathogenesis, we mined 421 genes in silico encoding putative transcription factors in a machine-annotated, draft genome sequence of A. brassicicola. In this study, targeted gene disruption mutants for 117 of the transcription factor genes were produced and screened. Three of these genes were associated with pathogenesis. Disruption mutants of one gene (AbPacC) were nonpathogenic and another gene (AbVf8) caused lesions less than half the diameter of wild-type lesions. Unexpectedly, mutants of the third gene, Amr1, caused lesions with a two-fold larger diameter than the wild type and complementation mutants. Amr1 is a homolog of Cmr1, a transcription factor that regulates melanin biosynthesis in several fungi. We created gene deletion mutants of Δamr1 and characterized their phenotypes. The Δamr1 mutants used pectin as a carbon source more efficiently than the wild type, were melanin-deficient, and more sensitive to UV light and glucanase digestion. The AMR1 protein was localized in the nuclei of hyphae and in highly melanized conidia during the late stage of plant pathogenesis. RNA-seq analysis revealed that three genes in the melanin biosynthesis pathway, along with the deleted Amr1 gene, were expressed at low levels in the mutants. In contrast, many hydrolytic enzyme-coding genes were expressed at higher levels in the mutants than in the wild type during pathogenesis. The results of this study suggested that a gene important for survival in nature negatively affected virulence, probably by a less efficient use of plant cell-wall materials. We speculate that the functions of the Amr1 gene are important to the success of A. brassicicola as a competitive saprophyte and plant parasite.

Cover crops and organic mulches for nematode, weed and plant health management

Summary – Traditional cover cropping systems for nematode management seldom consider weed and soil nutrient management concurrently. Integrating cover crops suppressive to plant-parasitic nematodes with a cover crop mulching system could improve traditional approaches. Two field experiments were conducted in 2003 and 2004 to evaluate ‘Tropic Sun’ sunn hemp (Crotalaria juncea) and ‘Iron Clay’ cowpea (Vigna unguiculata) as summer cover crops and as organic mulches. Both experiments were in a 3 × 3 split-plot design in which the main plots were summer planting of sunn hemp, cowpea or fallow, and the subplots were organic mulch of sunn hemp, cowpea or no mulch. The summer cover crop was followed by turnip (Brassica rapa) and lima bean (Phaseolus lunatus )i n the autumn. Using sunn hemp as organic mulch suppressed root-knot nematodes more effectively than using it as a cover crop, but only on a less susceptible host such as turnip, and not on a very susceptible host such as lima bean. While sunn hemp as a cover crop failed to enhance beneficial free-living nematodes, sunn hemp as an organic mulch enhanced bacterial-feeding nematode population densities. Sunn hemp mulch also suppressed broadleaf weeds but not grasses or nutsedges. Although sunn hemp and cowpea cover crops did not increase lima bean N and K content, their mulches increased N and K content. Similar results were observed for turnip and lima bean yields. Population density of root-knot nematodes was positively related to abundance of omnivorous nematode in 2003. The abundance of plant-parasitic nematodes was negatively related to the infestation levels of Pasteuria penetrans, and the abundance of predatory nematodes in 2004. Factors that might have affected the performance of sunn hemp on nematode communities are discussed.

Enhancement of Rotylenchulus reniformis suppressiveness by Crotalaria juncea amendment in pineapple soils

Abstract The abilities of sunn hemp, Crotalaria juncea amendment to enhance Rotylenchulus reniformis suppressiveness were tested in soils collected from six pineapple fields with different length of time after nematicides 1,3-dichloropropene application, and after pineapple planting. Soils were frozen to kill the indigenous nematodes, then either amended or not amended with chopped C. juncea tissues and infested with 1000 R. reniformis in a greenhouse experiment. Cowpeas were grown in these soils for 6 weeks. Amendment enhanced the population density of nematode-trapping fungi (NTF), the percentage of eggs parasitized by fungi, and the bacterivorous nematodes. Enhancement of NTF was highest in soils that had least been treated with 1,3-dichloropropene and in soils planted with pineapple for the longest period of time. NTF can also be enhanced by C. juncea in soils that had been fallowed without nematicides treatment. The suppression on R. reniformis eggs was correlated with the enhancement of NTF (r=0.65, P

Comparative Susceptibility of Two Banana Cultivars to Banana Bunchy Top Virus Under Laboratory and Field Environments

ABSTRACT Field and laboratory experiments were carried out on the island of Oahu, HI, to compare the susceptibility of the two most commonly grown banana (Musa sp.) cultivars in the state (‘Dwarf Brazilian’or Santa Catarina [locally known as dwarf apple] and ‘Williams’) to the aphid-borne Banana bunchy top virus (genus Babuvirus, family Nanoviridae, BBTV). Several morphological and physiological features of the two cultivars were monitored to determine whether the banana aphid, Pentalonia nigronervosa Coquerel (Hemiptera: Aphididae), transmits BBTV to both cultivars at a similar rate; and whether after successful inoculation, does each cultivar respond similarly to viral infection. Results from the laboratory experiment showed that a similar percentage of both cultivars were infected with BBTV by aphid vectors (>90% for both cultivars). However, field results showed a significantly lower percentage of dwarf apple (39%) infected with BBTV compared with Williams (79%). We also found that all physiological and morphological features measured (i.e., plant height, leaf area, canopy, chlorophyll level, and moisture content) for both cultivars were impacted similarly by BBTV. The incubation period, or the time between plant infection and initial appearance of disease symptoms, was similar for both cultivars. Results also showed that BBTV transmission efficiency was lower in the field than in the laboratory, despite that more aphids per plant were used for field than laboratory inoculation tests. The results highlight the potential use of less susceptible cultivars to help manage BBTV and the importance of screening banana varieties in the field to determine their response to vectors and associated diseases.

Aphid Transmission of Banana Bunchy Top Virus to Bananas After Treatment With a Bananacide

ABSTRACT Field and laboratory studies were conducted to determine the impact of using a herbicide as a bananacide on aphid transmission of Banana bunchy top virus (family Nanoviridae, genus Babuvirus, BBTV) to healthy banana (Musa spp.) plants. BBTV-infected banana plants in a commercial orchard were treated with Roundup Weathermax herbicide. Using polymerase chain reaction, the time after herbicide treatment that BBTV could no longer be detected in the infected plants was determined. The impact of the herbicide treatment on Pentalonia nigronervosa Coquerel (Hemiptera: Aphididae) virus acquisition and ability to inoculate healthy banana plants with BBTV also were determined. Generally, banana plants were dead beyond 42 d after herbicide injection (DAI), and BBTV was detected in a similar high percentage of treated plants from 0 up to 21 DAI. During two field trials, 0 and 32% of P. nigronervosa acquired the virus from treated plants at 42 DAI, respectively, but none successfully inoculated a healthy banana plant beyond 35 DAI. Finally, 22% of P. nigronervosa colonies collected directly from the pseudostem of injected plants at the final sample date (42 DAI) tested positive for BBTV and infected 9.5% of the healthy banana plants. The findings indicate that banana plants may remain a potential source of virus inoculum 6 wk after injection with a bananacide. The implications of these findings with respect to BBTV management are discussed.

Nonfumigant alternatives to methyl bromide for management of nematodes, soil-borne disease, and weeds in production of snapdragon (Antirrhinum majus)

Two field experiments were conducted in north Florida to examine effects of solarization and reduced-risk pesticides on weeds, nematodes, soil-borne diseases, and yield of snapdragon (Antirrhinum majus). Five treatments were replicated five times in a randomized complete block design: fumigation with 80:20 methyl bromide:chloropicrin, solarization, solarization + Kodiak® (product with Bacillus subtilis), solarization + Biophos™ (product with dipotassium phosphonate and dipotassium phosphate), and non-treated control. Methyl bromide was generally superior to treatments involving solarization, which in turn were superior to the control, for improving flower yield and managing weeds, nematodes, and mortality due to plant disease. In the second year, solarization was more effective than methyl bromide in reducing plant mortality; however, surviving plants were larger and more productive following methyl bromide. In the second year, Biophos™ + solarization was as effective as methyl bromide in improving plant height, but not yield. However, integrating solarization with Biophos™ did not provide benefits over solarization alone in terms of flower yield. In some cases, solarization alone provided results similar to methyl bromide for managing weeds, nematodes, and plant disease mortality.

In vitro screening for burrowing nematode, Radopholus citrophilus, tolerance and resistance in commercial Anthurium hybrids

SummaryA reliable method to screen Anthurium for burrowing nematode resistance and tolerance in vitro was developed using 17 genetically distinct Anthurium cultivars. Based on nonparametric data analysis, tolerance and resistance were found to be independent traits to be evaluated separately. An effective parameter for tolerance evaluation was ranking of relative leaf retention, whereas an effective parameter for resistance evaluation was the ranking of nematode reproduction, log(Rf+1). A comparison of the ranking of leaf retention with ranking of nematode reproduction clustered the cultivar responses to burrowing nematode infection into four groups: intolerant and resistant, moderately tolerant but susceptible, intolerant and susceptible, and tolerant and susceptible. ‘Ozaki’ was identified as an intolerant reference, ‘Nitta’ as a susceptible reference. ‘Blushing Bride’ was the most tolerant cultivar among those screened, but it may not be an ideal tolerant reference due to its low vigor. Future screening for burrowing nematode-tolerant and-resistant cultivars in Anthurium should include ‘Ozaki’ and ‘Nitta’ as internal controls. Evaluation of resistance should be based on a resistance index obtained by log(Rf of hybrid tested +1) divided by log(Rf of ‘Nitta’ +1); tolerance should be based on ranking of relative leaf retention.

Population Distribution and Density of Pentalonia nigronervosa (Hemiptera: Aphididae) Within Banana Mats: Influence of Plant Age and Height on Sampling and Management

ABSTRACT The banana aphid, Pentalonia nigronervosa Coquerel (Hemiptera: Aphididae), is the most economically important pest of banana (Musa spp.) fields in Hawaii. Recently, there has been a concerted effort in Hawaii to learn more about the biology and ecology of this pest. However, limited work has been directed at determining the distribution of P. nigronervosa in banana fields and developing an integrated pest management plan. Therefore, a survey was conducted in banana fields throughout the Hawaiian Islands to determine the distribution and density of P. nigronervosa within banana mats from plants of different stages. Another aim was to determine whether the presence of ants on banana plants could be used as a reliable indicator of aphid infestations. Results of the survey showed that plants ≤1.5 m (small sucker) in height contain the highest aphid populations per meter in plant height and that mother plants (≥2.5 m) had the lowest aphid counts and rate of infestation compared with small and intermediate suckers (>1.5 < 2.5 m). More specifically, aphid population was reduced by ≈12 aphids for every meter increase in plant height and that aphids are rarely found ≥2.5 m within the plant canopy. Although there was an increase likelihood of finding ants on banana plants with higher aphid densities, results suggest that ants would be present on plants in the absence of aphids. Implications of these and other findings with respect to sampling and managing P. nigronervosa and associated Banana bunchy top virus are discussed.

Growth and yield of zucchini squash (Cucurbita pepo L.) as influenced by a sunn hemp living mulch

The use of living mulches for managing crop pests has received much attention recently but their effects on primary crop productivity is relatively uninvestigated. This study investigated the influence of a sunn hemp (Crotalaria juncea L.) living mulch on growth, development and yield of zucchini squash (Cucurbita pepo L.). Field experiments were conducted at two locations during three growing seasons. Zucchini squash growth, dry biomass and yield were recorded and compared when inter-planted into a companion sunn hemp living mulch (SH) and grown in monoculture [bare-ground (BG)]. During the first and second study years, when sunn hemp was cut to a height of 45 cm and used concurrently as a living and surface mulch, zucchini plant dry biomass and yield were substantially reduced compared with BG habitats. However, in the third year, when sunn hemp was cut to a shorter height of 20 cm, zucchini plant growth and yield were similar between treatments at one study site and significantly greater in SH than in BG habitat at the other site. Potential causes of yield differences and implications of using sunn hemp as a living companion plant are discussed.

Suppression of Root-Knot Nematode by Vermicompost Tea Prepared From Different Curing Ages of Vermicompost

Suppression of root-knot nematodes (Meloidogyne spp.) by vermicompost tea (VCT) has been inconsistent. Greenhouse and laboratory trials were conducted to compare the effects of VCT prepared from different curing ages of vermicompost (VC) on root penetration, reproduction, and hatching of M. incognita. In the penetration experiment, zucchini (Cucurbita pepo) seedlings were drenched with VCT prepared from (i) uncured (UVC), (ii) partially cured (PVC), (iii) completely cured (CVC) vermicompost, and (iv) water or no vermicompost (NVC) 3 days prior to M. incognita inoculation. The experiment was repeated twice on cucumber (Cucumis sativus) and terminated one week after nematode inoculation. All three trials showed that UVC and PVC reduced (P ≤ 0.05) penetration of M. incognita compared with CVC and NVC. Two greenhouse trials showed that VCT from different curing ages of VC did not reduce the abundance of M. incognita juveniles in soil and eggs in roots 2.5 months after nematode inoculation. Two laboratory trials to examine hatching consistently showed that VCT from UVC and PVC suppressed hatching (P ≤ 0.05) compared with NVC, achieving 83.1% hatch reduction by UVC. Overall, VCT from UVC and PVC suppressed root penetration and hatching, but not the reproduction of M. incognita over time.