Feeding the Microbes: A Strategy to Control Verticillium Wilt

Verticillium dahliae is one of the most devastating soilborne pathogens for horticulture production. The pathogen has a broad host range and currently there is no effective chemical disease management, therefore, novel sustainable integrated disease management strategies should be considered. In this respect, we questioned whether the soil application of common microbiological growth media can influence the plant—microbe interactions and, subsequently, offer protection against V. dahliae. Indeed, the addition of Nutrient Broth (NB) and Potato Dextrose (PD) in non-sterilized soil reduced Verticillium wilt symptoms in eggplants. The addition of NB in sterilized soil did not reduce the disease symptoms compared to controls, however the addition of PD enhanced plant protection against V. dahliae. Following the results of a split root bioassay in eggplants, the possibility that NB and PD triggered the plant defense mechanisms against V. dahliae was excluded, since NB and PD did not reduce wilt symptom. Therefore, PD may be used as an easy food source for V. dahliae, detouring the pathogen from the root system of plants, while NB may affect the soil microbiome by enhancing antagonism in rhizosphere, or antagonistic interaction between V. dahliae and rhizospheric microbiome. Subsequently, several bacterial strains were isolated from the NB-treated rhizosphere and examined for their biocontrol activity against V. dahliae. Among the examined strains, a Pseudomonas putida strain, Z13, significantly reduced Veticillium severity and incidence under greenhouse conditions.

Screening of rootstock hybrids with Vitis cinerea Arnold for phylloxera resistance

The resistance to phylloxera is one of the most important properties of grapevine rootstock. The breeding and selection of rootstock varieties represents a biological method of fight against phylloxera. In this study altogether 59 rootstock hybrids originating from nine different populations were evaluated. Pedigrees of all populations involved Vitis cinerea Arnold introduced through the hybrid Bruci/(Vitis berlandieri Planch. × Vitis rupestris Scheele) × Vitis cinerea Arnold/ and the variety Börner. Pot experiments were performed in a glasshouse while those with excised root pieces in root bioassay took place in a laboratory. This combination showed to be a very good selection tool for rootstock breeding. In seven rootstock hybrids, it was found that the resistance to phylloxera gallicoles and radicicoles was very high. The highest number of resistant hybrids was found in Binova × Börner. The variety Börner showed to be the best gene source of phylloxera resistance and even in case of a simple crossing produced sufficient number of resistant progeny.

The development and reproduction of grape phylloxera (Daktulosphaira vitifoliae Fitch) strains of Hungarian vine-districts in root-bioassays

The root bioassay method allows for 4-6 week continuous observation of grape phylloxera feeding on the grape root. 10 Hungarian phylloxera strains were compared on susceptible Vitis vinifera cv. Chardonnay and the resistant rootstocks of V. berlandieri x V. riparia Teleki 5C and V. berlandieri x V. rupestris Georgikon 121 in in vitro observations. The strains originated from Villany and Eger (Figure 3) had higher reproduction on the root of V. berlandieri x V. riparia Teleki 5C (201 and 119 eggs) and were more aggressive than the others (average production 10 eggs). The continuous high level of survival, development and reproduction of the Eger strain on Teleki 5C (V. berlandieri x V. ripuria) until day 46 may be due to adaptation.

Phytotoxicity of Discula destructiva Culture Filtrates to Cornus spp. and the Relationship to Disease Symptomology

Discula destructiva culture filtrates and partially purified culture filtrates (PPCF) inhibited radish (Raphanus sativus) and dogwood (Cornus) species in a seedling root bioassay. Noninoculated potato-dextrose broth (PDB) extracted and separated in a similar manner also inhibited seedling growth. Reverse phase high performance liquid chromatography separated this inhibitory activity into two fractions, with one associated with the inhibitory action observed with PDB controls. The active fraction without interference with PDB, determined by bioassays, was extracted from cultures grown on Murishige-Skoog (MS) medium, which had no inhibitory activity associated with noninoculated controls. The active fraction was tested in a leaf overlay technique using 10 Cornus spp. All dogwood species were sensitive to the fraction and exhibited necrotic lesions bounded by a red margin, typical of dogwood anthracnose. The active fraction was translocated in Cornus alba to the leaf margin. C. canadensis showed minimal primary lesion formation but developed leaf curling and necrosis on leaf margins of newly emerging leaves, indicating apical translocation of the fraction from the application site. Comparison of three D. destructiva (Type 1) isolates and a Discula sp. (Type 2) isolate for production of the active fraction showed that the Type 2 isolate did not produce detectable amounts of the active component.

Contribution of Indole-3-Acetic Acid Production to the Epiphytic Fitness of Erwinia herbicola

ABSTRACT Erwinia herbicola 299R produces large quantities of indole-3-acetic acid (IAA) in culture media supplemented withl-tryptophan. To assess the contribution of IAA production to epiphytic fitness, the population dynamics of the wild-type strain and an IAA-deficient mutant of this strain on leaves were studied. Strain 299XYLE, an isogenic IAA-deficient mutant of strain 299R, was constructed by insertional interruption of the indolepyruvate decarboxylase gene of strain 299R with the xylE gene, which encodes a 2,3-catechol dioxygenase from Pseudomonas putidamt-2. The xylE gene provided a useful marker for monitoring populations of the IAA-deficient mutant strain in mixed populations with the parental strain in ecological studies. A root bioassay for IAA, in which strain 299XYLE inhibited significantly less root elongation than strain 299R, provided evidence that E. herbicola produces IAA on plant surfaces in amounts sufficient to affect the physiology of its host and that IAA production in strain 299R is not solely an in vitro phenomenon. The epiphytic fitness of strains 299R and 299XYLE was evaluated in greenhouse and field studies by analysis of changes in the ratio of the population sizes of these two strains after inoculation as mixtures onto plants. Populations of the parental strain increased to approximately twice those of the IAA-deficient mutant strain after coinoculation in a proportion of 1:1 onto bean plants in the greenhouse and onto pear flowers in field studies. In all experiments, the ratio of the population sizes of strain 299R and 299XYLE increased during periods of active growth on plant tissue but not when population sizes were not increasing with time.

Microbial degradation of imazaquin and imazethapyr

Laboratory studies were conducted to investigate the processes of imazaquin and imazethapyr degradation in soil. Microbial degradation of14C-imazaquin and14C-imazethapyr was monitored by measuring14CO2evolution compared to nonsterile soil.14CO2evolution was greatest from carboxyl-labeled imazaquin and imazethapyr compared to ring-labeled imazaquin and imazethapyr. A corn root bioassay indicated nearly complete loss of herbicidal activity in nonsterile soil after 5 mo, but herbicide activity was reduced only 14% in sterile soil.14CO2evolution more than doubled when soil temperature increased from 15 to 30 C. Total CO2production responded similarly. Degradation of imazaquin and imazethapyr increased as soil moisture increased from 15% (-2.4 MPa) to 75% of field capacity (-0.03 MPa).