Logistic regression modeling of prevalence of soybean sclerotinia stem rot in the north-central region of the United States

ABSTRACT Regional prevalence of soybean Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, was modeled using tillage practices, soil texture, and weather variables (monthly air temperature and monthly precipitation from April to August) as inputs. Logistic regression was used to estimate the probability of stem rot prevalence with historical disease data from four states of the north-central region of the United States. Potential differences in disease prevalence between states in the region were addressed using regional indicator variables. Two models were developed: model I used spring (April) weather conditions and model II used summer (July and August) weather conditions as input variables. Both models had high explanatory power (78.5 and 77.8% for models I and II, respectively). To investigate the explanatory power of the models, each of the four states was divided into small geographic areas, and disease prevalence in each area was estimated using both models. The R(2) value of the regression analysis between observed and estimated SSR prevalence was 0.65 and 0.71 for models I and II, respectively. The same input variables were tested for their significance to explain the within-field SSR incidence by using Poisson regression analysis. Although all input variables were significant, only a small amount of variation of SSR incidence was explained, because R(2) of the regression analysis between observed and estimated SSR incidence was 0.065. Incorporation of available site-specific information (i.e., fungicide seed treatment, weed cultivation, and manure and fertilizer applications in a field) improved slightly the explained amount of SSR incidence (R(2) = 0.076). Predicted values of field incidence generally were overestimated in both models compared with the observed incidence. Our results suggest that preseason prediction of regional prevalence would be feasible. However, prediction of field incidence would not, and a different site-specific approach should be followed.     

川芎茵核病的症状及病原菌鉴定

 Symptoms of sclerotinia stem rot of Ligusticum chuanxiong were observed and the causal agent of the disease was identified. The results showed that the symptoms of this disease began with the formation of dark brown hygrophanous splotch on the basal leaves, followed by the appearance of dark brown circular spot on the basal stem. Finally, the whole plant stoped growing and died. White mycelium and/or sclerotia were formed under moisturized conditions. On PDA medium, the pathogen of this disease formed brown sclerotia with various shapes and 2-5 mm in diameter. The diameter of apothecia was 0.3-0.8 cm and the length of stipes was 2-8 cm. Asci were columniform in shape and (100-135)×9μm in size, each containing 8 ascospores sized by (6-12) μm×(3-7)μm. No asexual spore formation was observed. Based on the morphological characteristics and the sequence of ribosomal DNAITS, the pathogen was identified as Sclerotinia sclerotiorum (Lib.) de Bary.     

Genetic structure of Phaeosphaeria nodorum populations in the north-central and midwestern United States

Stagonospora nodorum blotch, caused by Phaeosphaeria nodorum, is considered one of the most destructive foliar diseases of wheat in the United States. However, relatively little is known about the population biology of this fungus in the major wheat-growing regions of the central United States. To rectify this situation, 308 single-spore isolates of P. nodorum were analyzed from 12 populations, five from hard red spring wheat cultivars in Minnesota and North Dakota and seven from soft red winter wheat in Indiana and Ohio. The genetic structure of the sampled populations was determined by analyzing polymorphisms at five microsatellite or simple-sequence repeat (SSR) loci and the mating type locus. Although a few clones were identified, most P. nodorum populations had high levels of gene (H(S) = 0.175 to 0.519) and genotype (D = 0.600 to 0.972) diversity. Gene diversity was higher among isolates collected from spring wheat cultivars in North Dakota and Minnesota (mean H(S) = 0.503) than in those from winter wheat cultivars in Indiana and Ohio (H(S) = 0.269). Analyses of clone-corrected data sets showed equal frequencies of both mating types in both regional and local populations, indicating that sexual recombination may occur regularly. However, significant gametic disequilibrium occurred in three of the four populations from North Dakota, and there was genetic differentiation both within and among locations. Genetic differentiation between the hard red spring and soft red winter wheat production regions was moderate (F(ST) = 0.168), but whether this is due to differences in wheat production or to geographical variation cannot be determined. These results suggest that sexual reproduction occurs in P. nodorum populations in the major wheat-growing regions of the central United States, and that geographically separated populations can be genetically differentiated, reflecting either restrictions on gene flow or selection.     

Biocontrol of sclerotinia stem rot (Sclerotinia minor) in sunflower by seed treatment with Gliocladium virens

An isolate of Gliocladium virens from disease-affected soil in a commercial sunflower field proved highly antagonistic to Sclerotinia minor in culture. Significant disease control was obtained when a conidial suspension was applied over a limited area of potting medium and lateral roots were inoculated outside the treated area at the edge of the pot. Most promising, in terms of minimizing the amount of G. viren s required for disease control, was the significant bioprotection obtained by seed treatment. In glasshouse experiments following inoculation of lateral roots with S. minor at 6–7 weeks after sowing, disease incidence was reduced from 60.0% to 15.8% in potting medium and from 66.7% to 12.5% in field soil. Seed treatment was also effective in field microplots, where disease incidence was reduced from 37.0% to 22.2%. G. viren s applied on seed did not affect the dry weight of the lateral root system.     

The control of sclerotinia stem rot on oilseed rape (Brassica napus): current practices and future opportunities

Sclerotinia stem rot (SSR) caused by the phytopathogenic fungus Sclerotinia sclerotiorum is a major disease of oilseed rape (Brassica napus). During infection, large, white/grey lesions form on the stems of the host plant, perturbing seed development and decreasing yield. Due to its ability to produce long‐term storage structures called sclerotia, S. sclerotiorum inoculum can persist for long periods in the soil. Current SSR control relies heavily on cultural practices and fungicide treatments. Cultural control practices aim to reduce the number of sclerotia in the soil or create conditions that are unfavourable for disease development. These methods of control are under increased pressure in some regions, as rotations tighten and inoculum levels increase. Despite their ability to efficiently kill S. sclerotiorum, preventative fungicides remain an expensive gamble for SSR control, as their effectiveness is highly dependent on the ability to predict the establishment of microscopic infections in the crop. Failure to correctly time fungicide applications can result in a substantial cost to the grower. This review describes the scientific literature pertaining to current SSR control practices. Furthermore, it details recent advances in alternative SSR control methods including the generation of resistant varieties through genetic modification and traditional breeding, and biocontrol. The review concludes with a future directive for SSR control on oilseed rape.     

枯草芽胞杆菌NJ-18对油菜菌核病的防治效果及其定殖动态

 菌株NJ-18是从油菜田土壤中分离筛选到的一株具有抗真菌活性的枯草芽胞杆菌。试验结果表明菌株NJ-18与油菜菌核病菌对峙培养能形成3.4 cm的抑菌圈;能抑制油菜菌核病菌菌丝生长发育;强烈影响菌丝生长量,NJ-18发酵滤液稀释62.5倍时对菌丝生长量抑制率高达96.7 %,稀释1 000倍时抑制率16.7 %。油菜离体叶片试验表明,NJ-18发酵液原液对菌核病的防治效果高达100 %,原液稀释200倍后防效10 %。大田试验结果表明,NJ-18发酵液稀释500倍对油菜菌核病的防治效果高达57.4 %,明显高于用量为150 g a.i./hm²的醚菌酯48.1 %的防治效果。采用浸根法研究NJ-18在油菜上的定殖动态表明,菌株NJ-18能够在油菜体内定殖,并能从油菜的根部向叶子扩散。     

Bayesian logistic regression of soybean sclerotinia stem rot prevalence in the u.s. North-central region: accounting for uncertainty in parameter estimation

ABSTRACT Bayesian ideas have recently gained considerable ground in several scientific fields mainly due to the rapid progress in computing resources. Nevertheless, in plant epidemiology, Bayesian methodology is not yet commonly discussed or applied. Results of a logistic regression analysis of a 4-year data set collected between 1995 and 1998 on soybean Sclerotinia stem rot (SSR) prevalence in the north-central region of the United States were reexamined with Bayesian methodology. The objective of this study was to use Bayesian methodology to explore the level of uncertainty associated with the parameter estimates derived from the logistic regression analysis of SSR prevalence. Our results suggest that the 4-year data set used in the logistic regression analysis of SSR prevalence in the north-central region of the United States may not be informative enough to produce reliable estimates of the effect of some explanatory variables on SSR prevalence. Such confident estimations are necessary for deriving robust conclusions and high quality predictions.     

Management strategies and distribution of Aphanomyces root rot of alfalfa (Medicago sativa), a continuing threat to forage production in the United States

Alfalfa (Medicago sativa) is one of several legumes that is affected by Aphanomyces root rot (ARR) caused by Aphanomyces euteiches. Symptoms of ARR on alfalfa seedlings include a yellow-grey discolouration of roots, rotting and loss of lateral roots, stunted growth, chlorotic foliage and reduction of nitrogen-producing nodules on roots. Infection can also occur on adult plants leading to loss of lateral roots and nodules. At the seedling stage, ARR decreases alfalfa stand establishment, and field longevity is reduced when adult plants are infected. A. euteiches is an oomycete pathogen that has motile zoospores and thick-walled oospores that can survive for many years in soil. Two races are currently recognized by pathogenicity on differential alfalfa check cultivars. Most alfalfa cultivars contain race 1 resistance, but there is an increasing development of cultivars with resistance to race 2. Management strategies include planting resistant cultivars, avoiding planting in fields with poor drainage and rotating crops with nonhost plants.     

Fusarium stem and fruit rot of sweet peppers in the glasshouse

A stem and fruit rot of sweet peppers ( Capsicum annuum ) occurred in 1992 in West Sussex affecting a large proportion of plants of cv. Tasty and to a lesser extent cv. Mazurka. Fusarium solani was isolated from the stem lesions and also from a short distance above and below them. The disease was largely nodal. Distinctive fruit symptoms also occurred. Symptoms were reproduced by inoculation of wounds on the stem. On agar and in a small in vivo test the pathogen was sensitise to benomyl at 2 μg ml. The disinfectants, commercial sodium hypochlorite, Purogene and Tego 51 effectively killed macrospores following exposure for 15 min.     

Effect of inoculum density and soil tillage on the development and severity of rhizoctonia root rot

Rhizoctonia spp. cause substantial yield losses in direct-seeded cereal crops compared with conventional tillage. To investigate the mechanisms behind this increased disease, soils from tilled or direct-seeded fields were inoculated with Rhizoctonia spp. at population densities from 0.8 to 250 propagules per gram and planted with barley (Hordeum vulgare). The incidence and severity of disease did not differ between soils with different tillage histories. Both R. solani AG-8 and R. oryzae stunted plants at high inoculum densities, with the latter causing pre-emergence damping-off. High inoculum densities of both species stimulated early production of crown roots in barley seedlings. Intact soil cores from these same tilled and direct-seeded fields were used to evaluate the growth of Rhizoctonia spp. from colonized oat seeds. Growth of R. oryzae was not affected by previous tillage history. However, R. solani AG-8 grew more rapidly through soil from a long-term direct-seeded field compared to tilled soils. The differential response between these two experiments (mixed, homogenized soil versus intact soil) suggests that soil structure plays a major role in the proliferation of R. solani AG-8 through soils with different tillage histories.     

Use of reciprocal grafting to study brown stem rot resistance in soybean

ABSTRACT Resistance to brown stem rot of soybean has been identified and utilized in cultivar development and germ plasm enhancement. However, little is known about the nature of resistance to this disease. In an attempt to better understand the resistance mechanism by locating the tissue or tissues responsible for resistance, reciprocal grafts were made between brown stem rot-resistant and -susceptible genotypes including near-isogenic lines for resistance gene Rbs2. Results of brown stem rot evaluation of grafted plants indicate that brown stem rot resistance is conditioned by the root in all resistance sources evaluated.     

Characterization of Lasiodiplodia species causing leaf blight,stem rot and fruit rot of fig (Ficus carica) in Malaysia

Fig (Ficus carica) is an exotic deciduous plant that is grown worldwide. Fungal diseases pose a major threat to fig plants, affecting their fruit quality and production. This study was conducted to characterize the fungal isolates associated with leaf blight, stem rot and fruit rot of F. carica in Malaysia through morphological analysis, DNA sequencing, multigene phylogenetic analysis and pathogenicity tests. From September 2018 to March 2019, 30 blighted leaves and 30 rotted stems and fruits of F. carica were collected from several nurseries in Malaysia. Thirty fungal isolates that belonged to Lasiodiplodia theobromae (27 isolates) and L. brasiliensis (three isolates) were identified based on morphological characteristics, comparison of DNA sequences and phylogenetic analysis of the internal transcribed spacer (ITS), elongation translation factor 1-α (tef1-α), β-tubulin (tub2) and DNA-directed RNA polymerase II subunit (rpb2). Among the 27 isolates of L. theobromae, nine isolates were obtained from leaves, eight isolates from stems and 10 isolates from fruits, whereas the three isolates of L. brasiliensis were obtained from stems (two isolates) and a leaf (one isolate). The results of pathogenicity tests revealed that L. theobromae and L. brasiliensis isolates were responsible for leaf blight and stem rot of F. carica, whereas fruit rot was caused by L. theobromae isolates. The present study highlighted two different species, L. theobromae and L. brasiliensis, as the causal agents of leaf blight and stem rot of F. carica. Additionally, L. theobromae caused fruit rot of F. carica in Malaysia.     

密度、施肥、种植方式及杀虫剂处理对玉米穗腐病及伏马毒素污染的影响

为明确不同种植密度、施肥水平、耕作方式以及杀虫剂处理对玉米穗腐病发生及籽粒中伏马毒素污染水平的影响,于2014—2015年以浚单20、先玉335为供试玉米品种进行田间试验,通过改变种植密度、施肥水平、耕作方式及使用杀虫剂氯虫苯甲酰胺处理,在收获期调查玉米穗腐病的病情指数,采用高效液相色谱柱后衍生法检测玉米籽粒中伏马毒素含量。结果表明,先玉335和浚单20在田间发生穗腐病的病情指数及其籽粒中伏马毒素含量均随种植密度的增加而显著增大,在密度为7.5万株/hm~2时,其病情指数在2014年分别为16.00和10.33,在2015年分别为18.15和16.38,伏马毒素含量在2014年分别为2.38、1.53 mg/kg,在2015年分别为3.23、2.20 mg/kg,均显著高于密度为6.0万株/hm~2和4.5万株/hm~2时;穗腐病的病情指数及伏马毒素含量随着施肥量的增加而增大,在施肥量为675 kg/hm~2时,病情指数在2014年分别为12.35和8.37,在2015年分别为15.11和13.30,伏马毒素含量在2014年分别为1.35、0.85 mg/kg,在2015年分别为2.39、2.19 mg/kg,均显著高于施肥量为450 kg/hm~2及225 kg/hm~2时;耕作方式对穗腐病病情指数及伏马毒素含量无显著影响,旋耕后播种方式下穗腐病病情指数及伏马毒素含量略小于贴茬直播方式;使用杀虫剂氯虫苯甲酰胺处理后,能显著降低穗腐病发生的严重程度及伏马毒素的污染水平。     

Epidemiological aspects of Didymella bryoniae,the cause of stem and fruit rot of cucumber

The survival ofDidymella bryoniae and the incidence of ascospores in glasshouses, outdoors and under controlled conditions were studied. The fungus was able to overwinter in the open as dormant mycelium. Dry and undecomposed crop residues remained a source of infection for more than one year. Moisture and a minimum temperature between 5 and 10°C were needed for fructification. For ascospore release a high relative humidity was not sufficient, the substrate had to be moist during a short period. Ascospores could be trapped throughout day and night both outdoors and in glasshouses, but there was a marked peak during a period of 3 h in the evening. Both on days with and without rain about the same numbers of ascospores were trapped from crop residues in the open. Ascospore release was favoured by watering the plants in the glasshouse. Under controlled conditions the release of ascospores was determined by humidity and not by light or darkness. In a cucumber crop in the glasshouse the first ascospores were trapped at about the same time the first symptoms on the plants appeared. In the glasshouse with introduced diseased plant debris, particularly when the debris became wet when the plants were watered, the disease was more severe and yield was less than in a glasshouse without introduced plant debris. Airborne ascospores may cause the primary infection of a cucumber crop. Therefore, hygienic measures must be taken to eliminate plant debris as source of infection, both in glasshouses and outdoors.     

我国甘薯新病害-茎腐病的研究初报

 2006年以来广东省主要甘薯产区发现1种甘薯新病害,主要症状表现为叶片变黄,茎基部呈黑色水浸状腐烂,并逐渐沿茎枝向顶端腐烂,后整株倒伏、死亡。从甘薯病茎部分离得到了病菌,经柯赫法则验证为致病病原菌。根据病原菌的形态特征、培养性状、生理生化及16S rDNA序列分析,鉴定其为菊欧文氏菌(Erwinia chrysanthemi)。采用茎枝、菌液共培养法接种8个甘薯品种,结果表明8个品种均无抗病性,病原菌致病性较强。这是我国首次发现该病害。      

Resistance to cyfluthrin and tetrachlorvinphos in the lesser mealworm, Alphitobius diaperinus, collected from the eastern United States

The lesser mealworm, Alphitobius diaperinus (Panzer), is an important pest in poultry facilities. The toxicity of cyfluthrin and tetrachlorvinphos to five strains of the lesser mealworm was compared with the toxicity to a susceptible laboratory strain. Bioassays were carried out with both larvae and adults. For the susceptible strain, cyfluthrin and tetrachlorvinphos had similar toxicity to adults, but cyfluthrin was 5 times more toxic to larvae when compared with tetrachlorvinphos. High levels of resistance to tetrachlorvinphos in two beetle strains were detected in both larvae and adults, although these strains were heterogeneous and still contained susceptible individuals. Resistance to cyfluthrin ranged from 1.7- to 9.5-fold for adults and from 0.5- to 29-fold for larvae at the LC(95). Overall, the patterns of resistance did not mirror the insecticide use patterns reported at these facilities. The implications of these results to management of the lesser mealworms are discussed.     

黄葵茎腐病病原菌的分离与鉴定

针对江苏宜兴黄葵主产区发生的黄葵茎腐病,通过分离黄葵植株不同部位(根、茎、叶)以及根际土壤、根围土壤的微生物,经形态学观察,结合分子进化分析构建了黄葵不同生态位的微生物物种系统发育树,对其种类和分布进行了分析。结果表明,细菌优势类群为芽胞杆菌,而真菌无明显优势类群。同时,根据柯赫氏法则测定了分离得到的微生物菌株的致病性,确定了菌株F28可引起黄葵茎腐病,经ITS分子进化分析及形态学观察,将其鉴定为变紫青霉Penicillium sanguineum。     

Groundnut Rust in the United States and the Caribbean

Abstract

Groundnut rust, caused by Puccinia arachidis Speg., is endemic in the West Indies and has occurred with increasing frequency in the United States in recent years. Since 1970 this highly destructive fungus has become established throughout Asia and Oceania, in Australia, and in much of Africa. Rust, together with the leaf spots, greatly restricts production of groundnuts in the Caribbean and in Central America. However, it has not been a major cause of crop losses in the USA except in South Texas during 1971. The fungus does not overwinter in the USA but airborne uredospores are blown in from subtropical areas. Several fungicides currently recommended for leaf spot control are also effective against rust. Three distinct groundnut genotypes are known sources of marked resistance: Tarapoto (P.l.s 259747, 341879, 350680, 381622, 405132); Israel Line 136 (P.l.s 298115 and 315608); and DHT 200 (P.l. 314817). The resistance is physiological and apparently bigenic and recessive. Variation in reaction to different isolates suggests that races of groundnut rust occur. Rust-resistant lines with acceptable agronomic traits are under development in several breeding programmes.