A Non-destructive Leaf Disc Assay for Rapid Diagnosis of Weed Resistance to Multiple Herbicides

Weed resistance survey that monitors the spread of resistant weeds has been mainly conducted through time-consuming, labor-intensive and destructive greenhouse herbicide screens. As an alternative, we here introduce a non-destructive leaf disc assay based on chlorophyll fluorescence (Fv/Fm values which measure photosynthesis efficiency) allowing the detection of resistance to both systemic and contact herbicides within ˜48h. Current study validated the assay on detecting resistance to fomesafen, glyphosate, and dicamba in Palmer amaranth (Amaranthus palmeri S. Watson), waterhemp [Amaranthus tuberculatus (Moq.) Sauer], kochia [Bassia scoparia (L.) A.J. Scott] and goosegrass [Eleusine indica (L.) Gaertn.]. Negative correlation between the Fv/Fm values and the spray injury levels was observed in all herbicide-weed combinations at the discriminating doses, except for glyphosate in Amaranthus. The correlation coefficients were -0.41 for fomesafen (10µM, p<0.0001) in Amaranthus, -0.92 for glyphosate in E. indica (250µM, p<0.0001), and -0.44 for dicamba in B. scoparia (800 µM, p=0.0023), respectively. At the population level, the assay clearly separated susceptible from highly resistant populations. However, the assay showed lower sensitivity in distinguishing populations of different resistance levels or separating low resistance from susceptible populations. At the individual plant level, results from the leaf disc assay and whole plant spray tests were concordant in 85.5%, 92.3%, and 71.7% of the plants tested for fomesafen-Amaranthus, glyphosate-Eleusine and dicamba-Bassia, respectively. The assay yielded 1-15% false positive and 6-13% false negative results across herbicides. The current study demonstrated that the leaf disc assay is a useful tool to identify weed resistance. Optimization is needed to improve its sensitivities and expand its usage to more diverse herbicide-weed species combinations.

Occurrence and feeding preference of Diabrotica speciosa Germar and Cerotoma arcuata (Olivier) for different cultivars of cowpea Vigna unguiculata (Linnaeus) Walpers

Chrysomelid pests are not only known as leaf-feeding in beans but as vectors of viruses too. Chemical control is most commonly used but has high economic and environmental costs. This study aimed to report the occurrence of chrysomelid pests on cowpea area in the State of Tocantins and compare the feeding preference of the species collected in different cowpea cultivars. Weekly monitoring was carried out in cowpea stand at the experimental station of the Federal University of Tocantins, in Gurupi-TO. Insect feeding preference was evaluated under laboratory conditions (25 ± 2 °C, 60 ± 5% R.H.). A total of 1,542 adults of two species of chrysomelid pests was recorded: Cerotoma arcuata (Olivier) presented 71.49% and Diabrotica speciosa Germar, 28.51%. Leaf discs of the cultivars BRS "Tumucumaque"," BRS "Nova Era", BRS "Sempre Verde", and "Pingo de Ouro", were offered to adult beetles. It was observed a significant difference in no-choice leaf disc assays and four-choice leaf disc assays for the D. speciosa species, being BRS "Tumucumaque" the most consumed. In the no-choice leaf disc assay "Pingo de Ouro" was less consumed and showed non-preference type resistance. In the four-choice leaf disc assay, BRS Sempre Verde was more resistant to D. speciosa consumption. In both tests, C. arcuata did not show a significant difference among the tested cultivars, but mathematically, BRS "Nova Era" was the most consumed, while "Pingo de Ouro" the lowest consumed and showed non-preferred resistance in both tests. Cerotoma arcuata stood out as greater defoliant than D. speciosa in all cultivars tested.Ocorrência e preferência alimentar de Diabrotica speciosa Germar e Cerotoma arcuata (Olivier) por diferentes cultivares de feijão caupi Vigna unguiculata (Linnaeus) WalpersResumo. Crisomelídeos pragas não são apenas desfolhadores do feijoeiro, mas atuam também como vetores de viroses. O método de controle mais utilizado é o químico, o que acarreta altos custos econômicos e ambientais. Este trabalho objetivou registrar a ocorrência de crisomelídeos pragas em cultivo de feijão-caupi no estado do Tocantins, bem como comparar a preferência alimentar das espécies coletadas por diferentes cultivares. Foi realizado monitoramento semanal na área de feijão-caupi da estação experimental da Universidade Federal do Tocantins em Gurupi-TO. Foram registrados 1.542 adultos de duas espécies de crisomelídeos pragas: Cerotoma arcuata (Olivier)apresentou 71,49% e Diabrotica speciosa Germar, 28,51%. Em condições de laboratório, discos foliares das cultivares BRS "Tumucumaque", BRS "Nova Era", BRS "Sempre Verde" e "Pingo de Ouro" foram oferecidos a insetos adultos para testar sua preferência alimentar. Foi observada diferença significativa nos ensaios sem chance e com chance de escolha para espécie de D. speciosa, sendo o BRS "Tumucumaque" o mais consumido. Nos ensaios sem chance de escolha, "Pingo de Ouro" foi menos consumido e apresentou resistência do tipo não preferência. Nos ensaios com chance de escolha, o cultivar BRS "Sempre Verde" foi mais resistente ao consumo de D. speciosa. Em ambos os testes, C. arcuata não apresentou diferença significativa na preferência pelos quatro cultivares, embora BRS "Nova Era" tenha sido mais consumido em valores absolutos, enquanto "Pingo de Ouro" foi menos desfolhado apresentando resistência de tipo não preferência nos dois testes. Cerotoma arcuata destacou-se como maior desfolhador do que D. speciosa em todas os cultivares testados.

Evaluation of powdery mildew resistance in various melon (Cucumis melo L.) genotypes

Powdery mildew, caused by Podosphaera xanthii and Golovinomyces cichoracearum, is an economically important disease in melon worldwide. Genetic resistance is one of the most suitable strategies to control powdery mildew. During the last few years several races of the pathogens have been reported. The need to develop resistant varieties is a challenge for each breeding program. Leaf disc assay was used in phytopathology and breeding programs as a rapid and reliable method for evaluation of disease resistance in a large number of plant materials. The purpose of this study was to establish species and races of powdery mildew in Plovdiv region, South Central Bulgaria; to develop a suitable system of pathogen isolation and cultivation; to determine the resistance levels in different melon genotypes available in Maritsa Vegetable Crops Research Institute (MVCRI) - Plovdiv collection by the leaf disc assay. Fifty-three melon genotypes, including lines, varieties, hybrids and ten differential lines were tested. The data showed that causal agent of powdery mildew was race 1 of P. xanthii in Plovdiv region. Our experimental results indicated that for the long-term storage of powdery mildew it is preferable to keep a whole plant under in vitro conditions. This allows the preservation of powdery mildew for two months before transferring on a new tissue. Thirty-four of the tested melon genotypes reacted as immune or resistant and nineteen as susceptible. Resistant melon genotypes are a suitable source in initiating a new breeding program aimed to increase resistance to powdery mildew.

A leaf disc assay for determining resistance of taro to Phytophthora colocasiae

Phytophthora colocasiae is a destructive fungal pathogen of taro (Colocasia esculenta) which causes taro leaf blight This disease destroyed Samoas taro export industry and led to local food shortages following introduction of the pathogen in 1993 In support of a Pacific regional taro resistance breeding programme laboratory studies in New Zealand aimed to develop a leaf disc bioassay for identifying taro genotypes with disease tolerance A medium has been developed that will retard senescence of the leaf discs for over 14 days The rate of leaf disc infection and subsequent lesion development varied with incubation temperature leaf age and taro cultivar An understanding of these factors and the variation they cause has resulted in a laboratory leaf disc assay for testing resistance of different taro cultivars to taro leaf blight

Sensitivity of Erysiphe necator to Demethylation Inhibitor Fungicides in Virginia

Grape powdery mildew (Erysiphe necator) isolates were collected from 2005 to 2007 from vineyards mostly in Virginia but also some in Maryland, North Carolina, and Pennsylvania. Using a leaf disc assay, the isolates were tested against five demethylation inhibitor (DMI) fungicides. Most isolates exhibited reduced sensitivity to the five DMIs when compared with a sensitive group (n = 12) and compared with unexposed populations reported from other areas. The median resistance factor (RF) was highest for tebuconazole (RF = 399) and myclobutanil (RF = 378), followed by triflumizole (RF = 70), triadimefon (RF = 62), and fenarimol (RF = 44). The sensitive group used as the basis for comparison appears to have been more sensitive than unexposed isolates in New York and California. Our finding that the greatest resistance shift occurred with tebuconazole and myclobutanil contrasts with earlier reports from New York and California, where the greatest resistance shift was observed with triadimefon or triadimenol. Sensitivities to all five DMI fungicides were strongly correlated (pairwise r values of 0.70 to 0.87) but our data suggest that some may retain greater utility than others.

Sensitivity to fungicides in the isolates of Phytophthora infestans (Mont.) de Bary in the Czech Republic from 2003 to 2008

In the growing seasons from 2003 to 2008, 547 isolates of Phytophthora infestans from five regions in the Czech Republic were collected and examined for their sensitivity to the active ingredients (metalaxyl, dimethomorph and propamocarb-HCl) of frequently used fungicides. The response of the isolates to each of these substances was examined using the in vitro amended-agar method; in 352 of these isolates, the sensitivity to metalaxyl was also assessed by the floating leaf-disc assay. The majority of the isolates were sensitive (89.8%) to metalaxyl. Resistant isolates were found only in two of the sample years (2003 and 2008); they represented 58% of the samples in 2003 and only 29% in 2008. Four isolates from 2004 were found to be intermediate for their level of resistance. All the isolates that were tested were sensitive to dimethomorph and propamocarb-HCl; these particular substances completely suppressed mycelial growth at 1 &micro;g a.i. per ml.