Molecular Targets of Herbicides and Fungicides─Are There Useful Overlaps for Fungicide Discovery?

New fungicide modes of action are needed for fungicide resistance management strategies. Several commercial herbicide targets found in fungi that are not utilized by commercial fungicides are discussed as possible fungicide molecular targets. These are acetyl CoA carboxylase, acetolactate synthase, 5-enolpyruvylshikimate-3-phosphate synthase, glutamine synthase, phytoene desaturase, protoporphyrinogen oxidase, long-chain fatty acid synthase, dihydropteroate synthase, hydroxyphenyl pyruvate dioxygenase, and Ser/Thr protein phosphatase. Some of the inhibitors of these herbicide targets appear to be either good fungicides or good leads for new fungicides. For example, some acetolactate synthase and dihydropteroate inhibitors are excellent fungicides. There is evidence that some herbicides have indirect benefits to certain crops due to their effects on fungal crop pathogens. Using a pesticide with both herbicide and fungicide activities based on the same molecular target could reduce the total amount of pesticide used. The limitations of such a product are discussed.

Uptake of cephalexin by lettuce, celery, and radish from water.

The introduction of pharmaceuticals into agricultural lands from the application of biosolids and animal manure, and irrigation with treated wastewater has led to concern for animal and human health after the ingestion of pharmaceutical-tainted agricultural products. In this study, the uptake and accumulation of cephalexin, a commonly prescribed antibiotic, was compared in three common vegetables (lettuce, celery, and radish) grown in nutrient solution for 144 h. During the uptake experiments, cephalexin concentration in the nutrient solution decreased in the order of radish > celery > lettuce, while the accumulation of cephalexin in vegetable roots followed the rank of lettuce > celery > radish. The accumulation of cephalexin was below the limit of detection in radish roots. No accumulation of cephalexin was observed in the shoots of all three vegetables. The behaviors of cephalexin in vivo were further elucidated using in vitro measurements of cephalexin sorption by vegetable roots and transformation in plant enzyme extracts. The affinity of cephalexin to lettuce > celery > radish roots, and the respective sorption coefficients of 687, 303, and 161 mL g-1, coupled to the transformation of cephalexin in root enzyme extracts with estimated reaction rate constants of 0.020, 0.027 and 0.024 hr-1 for lettuce, celery and radish, could help elucidate the accumulation observed in the in vivo experiments. Overall, sorption by plant roots (affinity) and reaction with plant enzymes could collectively influence the uptake and accumulation of cephalexin in vegetables.

Changes in Winter Squash Fruit Exocarp Structure Associated with Age-Related Resistance to Phytophthora capsici.

Phytophthora capsici is a destructive pathogen of cucurbits that causes root, crown, and fruit rot. Winter squash (Cucurbita spp.) production is limited by this pathogen in Michigan and other U.S. growing regions. Age-related resistance (ARR) to P. capsici occurs in C. moschata fruit but is negated by wounding. This study aimed to determine whether structural barriers to infection exist in the intact exocarp of maturing fruit exhibiting ARR. Five C. moschata cultivars were evaluated for resistance to P. capsici 10, 14, 16, 18, and 21 days postpollination (dpp). Scanning electron microscopy imaging of Chieftain butternut fruit exocarp of susceptible fruit at 7 dpp and resistant fruit at 14 and 21 dpp revealed significant increases in cuticle and epidermal thicknesses as fruit aged. P. capsici hyphae penetrated susceptible fruit at 7 dpp directly from the surface or through wounds before 6 h postinoculation (hpi) and completely degraded the fruit cell wall within 48 hpi. Resistant fruit remained unaffected at 14 and 21 dpp. The high correlation between the formation of a thickened cuticle and epidermis in maturing winter squash fruit and resistance to P. capsici indicates the presence of a structural barrier to P. capsici as the fruit matures.

Mechanistic study on uptake and transport of pharmaceuticals in lettuce from water.

The dissemination of pharmaceuticals in agroecosystems originating from land application of animal manure/sewage sludge and irrigation with treated wastewater in agricultural production has raised concern about the accumulation of pharmaceuticals in food products. The pathways of pharmaceutical entries via plant roots, transport to upper fractions, and the factors influencing these processes have yet been systematically elucidated, thus impeding the development of effective measures to mitigate pharmaceutical contamination in food crops. In this study, lettuce uptake of thirteen commonly used pharmaceuticals was investigated using a hydroponic experimental setting. Pharmaceutical sorption by lettuce roots was measured in order to evaluate the influence on pharmaceutical transport from roots to shoots. Small-sized pharmaceuticals e.g., caffeine and carbamazepine with molecular weight (MW) <300 g mol-1 and a low affinity to lettuce roots (sorption coefficient Kp < 0.05 L g-1) manifested substantial transport to shoots. Small-sized molecules lamotrigine and trimethoprim had a relatively strong affinity to lettuce roots (Kp > 12.0 L g-1) and demonstrated a reduced transport to shoots. Large-sized pharmaceuticals (e.g. MW >400 g mol-1) including lincomycin, monensin sodium, and tylosin could be excluded from cell membranes, resulting in the predominant accumulation in lettuce roots. Large-sized oxytetracycline existed as zwitterionic species that could slowly enter lettuce roots; however, the relatively strong interaction with lettuce roots limits its transport to shoots. The mass balance analysis revealed that acetaminophen, ß-estradiol, carbadox, estrone and triclosan were readily metabolized in lettuce with >90% loss during 144-h exposure period. A scheme was proposed to describe pharmaceutical uptake and transport in plant, which could reasonably elucidate many literature-reported results. Molecular size, reactivity and ionic speciation of pharmaceuticals, as well as plant physiology, collectively determine their uptake, transport and accumulation in plants.

Metabolic Demethylation and Oxidation of Caffeine during Uptake by Lettuce.

Pharmaceuticals can be metabolized after being taken up by plants. The metabolites could manifest similar or equivalent bioactivity to the parent compound, promoting the critical need to understand the metabolism in plants. Caffeine has been frequently detected in agriculture produce; however, little attention is given to its metabolites in vegetables. This study examined uptake and metabolism of caffeine in lettuce in a hydroponic system. Caffeine and its metabolites in aqueous solution and lettuce were identified and quantified using a liquid chromatography coupled to a QTrap tandem mass spectrometry instrument. After 144 h, over 50% of applied caffeine dissipated in the hydroponic lettuce system, and eight caffeine metabolites were identified primarily in the shoots. Caffeine underwent demethylation reactions, which were confirmed with authentic standards, and the total amount accounted for 20% of the initially applied caffeine. Other metabolism pathways included oxidation and hydroxylation, and the amount of metabolites increased over uptake time.

Characteristics of Resistance to Phytophthora Root and Crown Rot in Cucurbita pepo.

Root and crown rot incited by Phytophthora capsici causes considerable annual losses in squash-producing regions in the United States. 'Spineless Perfection' zucchini and 'Cougar' straightneck squash (Cucurbita pepo L.), partially resistant and susceptible to root and crown rot, respectively, were investigated for differences in root and crown physical factors and the histology of crown infection by P. capsici. The pH and titratable acidity of healthy root and crown tissue from tissue extracts were not significantly different between cultivars (P = 0.05). Crude fiber content (%) of blended and oven-dried root and crown tissue from healthy plants was similar between cultivars. However, dry matter (%) was highest for Cougar (P = 0.05). Colonies of P. capsici grown from mycelial plugs in root exudates collected from each cultivar were similar in diameter. Whole mounts and histological sections of healthy and infected crown tissue revealed that vascular bundles and metaxylem vessels were more abundant in crowns of Spineless Perfection than Cougar. Twelve to 48 h post inoculation (hpi), mycelia in the crown of each cultivar was limited to the cortex and hypodermal tissue. By 72 hpi, hyphae were observed in the cortex and endodermal tissue of Cougar and were concentrated in the phloem and parenchyma cells surrounding vascular bundles. Mycelia were limited to the outer cortex in Spineless Perfection. Mycelia and occluding material were present in the majority of metaxylem vessels of Cougar but not Spineless Perfection at 92 hpi; dissolution of parenchyma cells surrounding vascular bundles was apparent in Cougar. The vascular occlusions observed in Cougar may be responsible for plant wilting, a common disease symptom. Additional straightneck, crookneck, scallop, and acorn squash (C. pepo ssp. ovifera), and zucchini, marrow, and pumpkin (C. pepo ssp. pepo) cultivars were evaluated in a greenhouse study for resistance to root and crown rot. Cucurbita pepo ssp. ovifera cultivars were significantly more susceptible than C. pepo ssp. pepo to root and crown rot (P < 0.0001). Growing C. pepo ssp. pepo cultivars may be beneficial in an integrated Phytophthora management program.

The National Plant Diagnostic Network: Partnering to Protect Plant Systems.

The National Plant Diagnostic Network (NPDN) has developed into a critical component of the plant biosecurity infrastructure of the United States. The vision set forth in 2002 for a distributed but coordinated system of plant diagnostic laboratories at land grant universities and state departments of agriculture has been realized. NPDN, in concept and in practice, has become a model for cooperation among the public and private entities necessary to protect our natural and agricultural plant resources. Aggregated into five regional networks, NPDN laboratories upload diagnostic data records into a National Data Repository at Purdue University. By facilitating early detection and providing triage and surge support during plant disease outbreaks and arthropod pest infestations, NPDN has become an important partner among federal, state, and local plant protection agencies and with the industries that support plant protection.

Glyphosate effects on plant mineral nutrition, crop rhizosphere microbiota, and plant disease in glyphosate-resistant crops.

Claims have been made recently that glyphosate-resistant (GR) crops sometimes have mineral deficiencies and increased plant disease. This review evaluates the literature that is germane to these claims. Our conclusions are: (1) although there is conflicting literature on the effects of glyphosate on mineral nutrition on GR crops, most of the literature indicates that mineral nutrition in GR crops is not affected by either the GR trait or by application of glyphosate; (2) most of the available data support the view that neither the GR transgenes nor glyphosate use in GR crops increases crop disease; and (3) yield data on GR crops do not support the hypotheses that there are substantive mineral nutrition or disease problems that are specific to GR crops.

Identification and onion pathogenicity of Burkholderia cepacia complex isolates from the onion rhizosphere and onion field soil.

Burkholderia cepacia complex strains are genetically related but phenotypically diverse organisms that are important opportunistic pathogens in patients with cystic fibrosis (CF,) as well as pathogens of onion and banana, colonizers of the rhizospheres of many plant species, and common inhabitants of bulk soil. Genotypic identification and pathogenicity characterization were performed on B. cepacia complex isolates from the rhizosphere of onion and organic soils in Michigan. A total of 3,798 putative B. cepacia complex isolates were recovered on Pseudomonas cepacia azelaic acid tryptamine and trypan blue tetracycline semiselective media during the 2004 growing season from six commercial onion fields located in two counties in Michigan. Putative B. cepacia complex isolates were identified by hybridization to a 16S rRNA gene probe, followed by duplex PCR using primers targeted to the 16S rRNA gene and recA sequences and restriction fragment length polymorphism analysis of the recA sequence. A total of 1,290 isolates, 980 rhizosphere and 310 soil isolates, were assigned to the species B. cepacia (160), B. cenocepacia (480), B. ambifaria (623), and B. pyrrocinia (27). The majority of isolates identified as B. cepacia (85%), B. cenocepacia (90%), and B. ambifaria (76%) were pathogenic in a detached onion bulb scale assay and caused symptoms of water soaking, maceration, and/or necrosis. A phylogenetic analysis of recA sequences from representative B. cepacia complex type and panel strains, along with isolates collected in this study, revealed that the B. cenocepacia isolates associated with onion grouped within the III-B lineage and that some strains were closely related to strain AU1054, which was isolated from a CF patient. This study revealed that multiple B. cepacia complex species colonize the onion rhizosphere and have the potential to cause sour skin rot disease of onion. In addition, the onion rhizosphere is a natural habitat and a potential environmental source of B. cenocepacia.

Effect of Treating Apple Trees with Acibenzolar-S-Methyl on Fire Blight and Expression of Pathogenesis-Related Protein Genes.

Acibenzolar-S-methyl (ASM, Actigard 50 WG), a synthetic inducer of systemic acquired resistance (SAR) and pathogenesis-related (PR) proteins, was evaluated for the control of fire blight on apple trees in the field and for PR protein gene expression in apple seedlings in the greenhouse. Expression of putative genes related to SAR induction was elevated in ASM-treated apple seedlings 2 to 7 days after treatment; levels of PR-1 and PR-8 RNA were increased 10-fold and PR-2 RNA was increased 100-fold in ASM-treated seedlings over levels in untreated seedlings. Spurs and shoots on Jonathan trees sprayed with ASM at 75 mg a.i./liter at pink and at weekly intervals thereafter had significantly less fire blight than untreated trees, both from natural infection and following artificial inoculation with the pathogen. Less fire blight was observed on trees sprayed weekly with ASM than on trees sprayed biweekly. The severity of fire blight on inoculated shoots of Fuji apple decreased with increasing rates of ASM (0 to 300 mg a.i./liter), and ASM combined with streptomycin showed enhanced activity over ASM at 75 mg a.i./liter alone. In general, ASM was not superior to streptomycin for fire blight control, but integrating a weekly schedule of ASM, preferably at 150 mg a.i./liter, with a schedule of streptomycin designed for blossom blight control appears promising for overall improvement in fire blight control.