Disease Progress Curve Values Research Articles

Screening of onion (Allium cepa) genotypes to find out novel resistant source against purple blotch (Alternaria porri)

Purple blotch caused by Alternaria porri (Ellis) Cif., 1930 causes huge losses to bulb crop and seed crop of onion (Allium cepa L.). This is a major disease affecting successful onion production in majority of the onion growing areas. To identify resistant source against purple blotch, 34 onion genotypes with good marketable yield based on field evaluation trial in 2017 and allied species Pusa Soumya were screened under protected conditions during kharif, 2018 at insect proof net house facility of ICAR-Indian Agricultural Research Institute, New Delhi. The pure culture of Alternaria porri, isolated from infected leaf samples of onion crop raised under natural epiphytotic conditions during kharif, 2017. Highest change of disease severity index was observed between 1–2 weeks post inoculation (WPI) while highest disease severity index was recorded at 5 WPI suggesting that preventive measures must be employed as soon as disease symptoms appear in the field. The disease severity index ranged from 0.10–33.33 at 1 WPI, which increased to 9.33–89.33 at 5 WPI. The cultivated allied species Pusa Soumya was identified as novel resistance source with DSI value less than 10.0% at 5 WPI. Four genotypes exhibited moderately susceptible reaction, while 20 genotypes were susceptible and 10 genotypes were highly susceptible. The area under disease progress curve (AUDPC) value ranged from 22.33–240.17. Pusa Soumya had the lowest value and among onion lines Arka Kirtiman was found promising. Three onion genotypes had low relative AUDPC value and can effectively be utilized in resistance breeding programme. The average daily temperature between 20–25°C and relative humidity above 75% was found ideal for disease development.

Multi-environment evaluation of rice genotypes: impact of weather and culm biochemical parameters against sheath blight infection.

Sheath blight caused by Rhizoctonia solani is one of the major diseases of rice, causing widespread crop losses. The use of semi-dwarf rice varieties in the ongoing nutrient-intensive rice cultivation system has further accentuated the incidence of the disease. An ideal solution to this problem would be identifying a stable sheath blight-tolerant genotype. A multi-environment evaluation of 32 rice genotypes against sheath blight infection was conducted over six seasons across two locations (Agricultural Research Farm, Institute of Agricultural Sciences, Banaras Hindu University (28.18° N, 38.03° E, and 75.5 masl), for four years during the wet seasons (kharif) from 2015 to 2018 and two seasons at the National Rice Research Institute (20°27'09" N, 85°55'57" E, 26 masl), Cuttack, Odisha, during the dry season (rabi) of 2019 and the kharif of 2019, including susceptible and resistant check. Percent disease index data were collected over 4 weeks (on the 7th, 14th, 21st, and 28th day after infection), along with data on other morphological and physiological traits. The resistant genotypes across seasons were the ones with a higher hemicellulose content (13.93-14.64) and lower nitrogen content (1.10- 1.31) compared with the susceptible check Tapaswini (G32) (hemicellulose 12.96, nitrogen 1.38), which might explain the resistant reaction. Three different stability models-additive main effect and multiplicative interaction (AMMI), genotype + genotype x environment (GGE) biplot, and multi-trait stability index (MTSI)-were then used to identify the stable resistant genotypes across six seasons. The results obtained with all three models had common genotypes highlighted as stable and having a low area under the disease progress curve (AUDPC) values. The ideal stable genotypes with low disease incidence were IC 283139 (G19), Tetep (G28), IC 260917 (G4), and IC 277274 (G10), with AUDPC values of 658.91, 607.46, 479.69, and 547.94, respectively. Weather parameters such as temperature, rainfall, sunshine hours, and relative humidity were also noted daily. Relative humidity was positively correlated with the percent disease index.

Differential susceptibility of two Citrullus amarus pollenizer watermelons to five species of Pythium and Globisporangium.

Pollenizer watermelon (Citrullus amarus) must be planted alongside triploid watermelon cultivars to provide a source of pollen. Early season death of pollenizer watermelon cultivars SP-6 and SP-7 was observed in a research field in Charleston County, South Carolina, in April 2022, 3 to 4 weeks after transplanting. Disease incidence was 15% and 12% for the two cultivars, respectively. Two species of Pythium, P. myriotylum (four isolates) and P. aphanidermatum (three isolates), and three species of Globisporangium, G. ultimum (one isolate), G. irregulare (one isolate), and G. spinosum (four isolates), were recovered from 13 of 17 plants cultured and identified based on the sequences of the cytochrome c oxidase subunit I gene. Pythium spp. were recovered from both cultivars, while Globisporangium spp. were recovered only from SP-7. In pathogenicity tests, SP-7 had lower area under the disease progress curve (AUDPC) values than SP-6 with one isolate each of four species and lower AUDPC values than seedless watermelon Estrella with one isolate each of two species. Mean AUDPC values did not differ between Estrella and SP-6 with any isolate. AUDPC values were greater with G. ultimum on all three cultivars than with one isolate of G. spinosum and both isolates of P. myriotylum and P. aphanidermatum. Susceptibility of the new cultivar SP-7 to Pythium and Globisporangium spp. differs from susceptibility of SP-6. These results will help growers choose which pollenizer cultivar to plant.

Defense-related gene expression in response to the application of biological control agents in banana

Black Sigatoka, caused by P. fijiensis, is the most important banana disease in Latin America as increasing applications of chemical fungicides used for its management result in both higher banana production costs and pathogen resistance. Spores of Bacillus subtilis, a proprietary Trichoderma sp., and a biological fermentation product composed of Saccharomyces cerevisiae cell walls and copper sulfate, were applied separately to evaluate potential defense responses in banana plants. The leaves were artificially inoculated with the pathogen, and the biological control agents were applied to the soil to avoid direct antagonism with the pathogen. Significantly lower (P < 0.05) area under disease progress curve values were observed among all treated plants compared with the control, with the lowest value measured in the treatment with a 5 % (w/v) concentration of a yeast cell wall product. In a second trial, the same biological control agents were applied to banana plants at 5 % (w/v) concentration to determine the relative expression of defense-related genes. Higher relative expression (P < 0.05) of PR3 and PR9 was observed at 3 and 24 h after application in the plants treated with B. subtilis and Trichoderma sp., respectively. In the plants treated with the yeast cell wall product, higher relative expression (P < 0.05) of PR3 was measured 48 h after application. The findings indicate that plant defenses may be activated by the biological control agents.

Relationship of weather parameters and date of showing on vector-borne disease, stem necrosis of potato

Stem necrosis is a hot spot disease caused by Tospovirus, a groundnut bud necrosis virus strain for Rajasthan. The minimum per cent incidence (5.56%) of stem necrosis was recorded with the variety Kufri Sindhuri and Kufri Pushkar (7.69%), while the maximum per cent incidence (24.87%) in Kufri Pukhraj and Kufri Bahar (23.75%). Out of three sowing dates, minimum disease incidence was recorded on 3rd planting date. The maximum and minimum temperature was highly significant, with a positive correlation against stem necrosis on the first planting date. A highly significant positive correlation between stem necrosis and the thrips population was detected. The maximum area under the disease progress curve (AUDPC) value was found on 1st date of sowing. The 3rd date of sowing produced the maximum number of tubers per plot, marketable yield and total tuber yield (q/ha). However, the maximum per cent dry matter was recorded in K. Sindhuri (22.27%) and K. Pushkar (21.01%). Thus, the experiment results suggested that the variety of Kufri Sindhuri and Kufri Pushkar with planting in the last week of October caused a significant reduction in stem necrosis disease leading to a corresponding increase in tuber yield of potato.

Replacing Low-Risk Fungicides with High-Risk Fungicides for Control of Powdery Mildew in a Resistant and Susceptible Summer Squash Cultivar in New Jersey

From 2016 to 2018, six fungicide programs for the control of cucurbit powdery mildew ( Podosphaera xanthii) on a powdery mildew-resistant summer squash ‘Reward F1’ and powdery mildew-susceptible ‘Senator’ were evaluated with an emphasis on reducing the number of chlorothalonil applications. In all 3 years, cucurbit powdery control was greatest (based on lowest area under disease progress curve values) when fungicides with a high risk (HR) for resistance development and with different modes of action were applied in rotation all season or when HR fungicides were applied before and following the onset of disease development, rather than weekly following symptom observation in the susceptible and resistant cultivars. For a disease such as cucurbit powdery mildew, where there are multiple HR fungicides with different modes of action available and where their use could be limited to one or two applications per growing season, these fungicides might be suitable replacements for protectant fungicides in cucurbit crops that have stable genetic resistance to cucurbit powdery mildew.

High Input of Nitrogen Fertilization and Short Irrigation Frequencies Forcefully Promote the Development of Verticillium Wilt of Olive.

It is known that high N doses, N/K imbalances, and frequent irrigation favor Verticillium wilt. The influence of fertilization and its interaction with the frequency of irrigation on the development of Verticillium wilt of olive (VWO) has been evaluated. A split-split-plot design in microplots with two naturally infested soils of different texture was established for studying three fertilization treatments (NO3Ca, NPK and without fertilization), plus two irrigation frequencies (daily and deficit). The treatments were applied by means of fertigation, evaluating the susceptible cultivar Picual. Final disease incidence in plants subjected to NO3Ca daily treatment was 100% regardless of soil texture. However, final mortality in these plants was 37% and 85.2% in clay and sandy loam soils, respectively. In addition, the area under the disease progress curve values were significantly higher (49.1%) in plants subjected to NO3Ca fertilization compared to those not fertilized or fertilized with N-P-K when plants were grown in clay soil. This value in the sandy loam soil was significantly higher in the NO3Ca daily irrigation treatment (94.3%), followed by the N-P-K-daily treatment (61.1%) which also was significantly higher than the unfertilized daily, N-deficit and NPK-deficit treatments (37.8, 42.6 and 44.9%, respectively). The plants submitted to unfertilized-deficit treatment reached the lowest value (9.6%). In this work it can be concluded that the application of fertilizer or the application of fertilizer with daily irrigation in naturally infested soils increases the development of VWO in Picual.

Management of powdery mildew and aphid in organically grown Indian mustard (Brassica juncea L.) through different oils

All the cultivated Brassica species exhibited powdery mildew severity with highest (43.2%) on Brassica juncea followed by B. rapa (28.0%) which is widely cultivated in India. Yield losses from 10-90 per cent with reduction in 6.47 per cent oil content have been estimated from different Brassica species. Hence, the present study was carried out to evaluate different oils to manage powdery mildew disease and aphids in an organically grown mustard crop. Five oils (castor, groundnut, sunflower, neem and mustard oil) and ghee were evaluated against powdery mildew (Erysiphae cruciferarum) and aphid (Lipaphis erysimi) of Indian mustard (Brassica juncea). Castor oil 50 EC, neem oil 50 EC and mustard oil 50 EC and deshi ghee 50 EC (@ 100 ml/10 litre water) were effective over untreated control in reducing powdery mildew disease severity when applied as foliar spray (@ 5 ml/litre) in the experiment carried out in organic field (plot No. B-2 at Agronomy Instruction Farm, S. D. Agricultural University, Sardarkrushinagar). Maximum reduction in germinated conidia (66.0%) over control, disease severity (50.0%), area under disease progress curve (AUDPC) values (51.8%) and maximum increase over control in 1000 seed weight (16.7%) and seed yield (10.3%) was observed with neem oil. This was followed by the application of ghee which exhibited reduced disease severity (22.9%) and AUDPC values (30.6%) and increased seed yield (8.1%) and the application of castor oil 50 EC which exhibited an increased in seed yield (8.0%) and reduced powdery mildew severity (20.8%). Shortest log phase was observed with neem oil during 10th std. week. All the oils tested (castor, groundnut, sunflower, neem and mustard oil) and deshi ghee were effective in reducing aphid population (39.5 to 65.9%) and damage index (6.1 to 25.1%). A maximum reduction in aphid population (65.9%) over control and lowest aphid damage index (3.70) was observed with neem oil 50 EC 6 days after 2nd spray. Application of neem oil, castor oil and deshi ghee resulted in increased seed yield (8.1 to 10.4%). This information could be useful in devising strategies for powdery mildew and aphid management in organic cultivation, eco-friendly management and IPM of mustard.

Phenotypic Screening of F3 Rice (Oryza sativa L.) Population Resistance Associated with Sheath Blight Disease

The experiment was conducted during July–December, 2019 in the Agricultural Research Farm Institute of Agricultural Sciences, Banaras Hindu University, Varanasi, Uttar Pradesh, India. One hundred and twenty-two F3 rice populations from the cross made between IC277332 (susceptible parent)×IC277275 (moderately resistant parent) were evaluated against sheath blight disease under field conditions. The area under disease progress curve (AUDPC) values categorized rice population into four groups viz., moderately resistant (24), moderately susceptible (38), susceptible (40), and highly susceptible (20). A moderate resistance level to the disease was identified during the study in 24 lines (19.6%) with a mean Percentage Disease Index (PDI) of 12.22%−36.6%. Furthermore, 38 individuals showed moderate susceptibility with AUDPC values (1012−1446 day-1). The maximum PDI and AUDPC value were 76.11 and 2325.56, and the minimum PDI and AUDPC values were 22.78 and 622.22, respectively. The principal component (PCA) biplot analysis showed 71.62% and 12.05% variation, respectively. Unweighted Pair Group Method of Arithmetic Means (UPGMA) cluster analysis grouped the 122 individuals into two major clusters, A and B and sub-clusters. These findings indicated that no rice line resistant to sheath blight had been identified. However, few population lines exhibited moderate resistance, which can be utilized as donor lines to generate sheath blight-resistant rice cultivars. These findings will provide a solid basis for our future breeding and screening activities at the institution.

Varietal Screening of Durum Wheat Varieties for Resistance to Pyrenophora tritici-repentis (Tan Spot) under Field Conditions.

Tan spot disease caused by Pyrenophora tritici-repentis was becoming more bred in Tunisia during the last decade. The search for resistant varieties against the increased virulence diversity of P. tritici-repentis is presently considered as a priority. Seven of the most commercialized durum wheat varieties in Tunisia (cvs. Maâli, Salim, Razzak, Monastir, Khiar, Inrat100, and Sculptur) were inoculated with five characterized fungal strains under field conditions, during two seasons. The variance analysis revealed that strains Ech8F6 and B4.8 used in inoculation are the most virulent ones. These strains hosting ToxB gene caused chlorosis symptom on the tested varieties. The other strains induced necrosis with yellow halo and host ToxA gene were less virulent. The area under disease progress curve values revealed that Maâli is the most vulnerable genotype compared to the new selected varieties Monastir and Inrat100. A variable tolerance rate of the varieties to tan spot disease was also highly visible on yield components. The losses were about 22.2% of the thousand kernel weight in Maâli variety, 35% of spikes/m2 in Inrat100 variety, 32.5% of kernel number/spike, and 25.2% of yield grain in Monastir variety. This effect evaluation of the strains harbouring ToxA and ToxB genes could be responsible for the identification of potentially susceptible genes Tsn1 and Tsc2 representing resistance sources for breeding programs.

Duration of Downy Mildew Control Achieved with Fungicides on Cucumber Under Florida Field Conditions.

Cucurbit production in Florida is impacted by downy mildew on a yearly basis. Cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, is one of the most devastating cucurbit diseases and can lead to complete yield loss. Nearly continuous production of cucurbits occurs temporally throughout Florida, which puts extensive pressure on the pathogen population to select for individuals that are resistant to fungicides in use labeled for CDM. Loss of efficacy as a result of fungicide resistance developing is becoming a major concern for Florida cucurbit growers who rely on these products to manage CDM. This study was established to evaluate the field activity of 11 utilized fungicides by determining their duration of activity when applied at various intervals for the management of CDM in cucumber under Florida field conditions. By comparing levels of percent CDM control and area under the disease progress curve values, the fungicide's duration of field activity was established. Field activities were <1 week for dimethomorph and fluopicolide; 1 week for cymoxanil; 1 to 2 weeks for chlorothalonil and mancozeb; 2 weeks for ethaboxam; 1 to 3 weeks for propamocarb, cyazofamid, and ametoctradin + dimethomorph; and 2 to 4 weeks for oxathiapiprolin and fluazinam. Knowledge of duration of field activity can potentially improve the development of CDM management programs and slow the resistance selection.

Evaluation of F2:3 rice population resistant to Rhizoctonia solani Kuhn inciting sheath blight disease

Rice sheath blight caused by the soil-borne fungal pathogen Rhizoctonia solani Kuhn is an economically important disease in rice resulting in enormous yield losses worldwide. In the present investigation, a population constituting F3 lines resulted from the cross made between IC277332 (susceptible parent) and Tetep (resistant parent) were evaluated for sheath blight resistance and other agronomic traits over a season. The rice population lines were categorized into four groups viz., moderately resistant (11), moderately susceptible (63), susceptible (24), and highly susceptible (8), based on area under disease progress curve (AUDPC) values. During the study, nine moderate resistant lines showed, less AUDPC values in comparison to Tetep. Furthermore, 63 individuals (60%) exhibited moderate susceptibility with AUDPC values (677-987 per day). The principal component biplot analysis PC1 and PC2 showed 47.08% and 13.19% variation, respectively. The employment of Unweighted Pair Group Method of Arithmetic Means (UPGMA) cluster analysis led to the grouping of the 106 individuals into 2 major clusters A and B. The results suggested that none of the rice lines was resistant to sheath blight disease. However, few lines showed moderate resistance to the disease which can be exploited for the development of sheath blight-resistant cultivars.

Discovery and Validation of a Recessively Inherited Major-Effect QTL Conferring Resistance to Maize Lethal Necrosis (MLN) Disease.

Maize lethal necrosis (MLN) is a viral disease with a devastating effect on maize production. Developing and deploying improved varieties with resistance to the disease is important to effectively control MLN; however, little is known about the causal genes and molecular mechanism(s) underlying MLN resistance. Screening thousands of maize inbred lines revealed KS23-5 and KS23-6 as two of the most promising donors of MLN resistance alleles. KS23-5 and KS23-6 lines were earlier developed at the University of Hawaii, United States, on the basis of a source population constituted using germplasm from Kasetsart University, Thailand. Both linkage mapping and association mapping approaches were used to discover and validate genomic regions associated with MLN resistance. Selective genotyping of resistant and susceptible individuals within large F2 populations coupled with genome-wide association study identified a major-effect QTL (qMLN06_157) on chromosome 6 for MLN disease severity score and area under the disease progress curve values in all three F2 populations involving one of the KS23 lines as a parent. The major-effect QTL (qMLN06_157) is recessively inherited and explained 55%–70% of the phenotypic variation with an approximately 6 Mb confidence interval. Linkage mapping in three F3 populations and three F2 populations involving KS23-5 or KS23-6 as one of the parents confirmed the presence of this major-effect QTL on chromosome 6, demonstrating the efficacy of the KS23 allele at qMLN06.157 in varying populations. This QTL could not be identified in population that was not derived using KS23 lines. Validation of this QTL in six F2 populations with 20 SNPs closely linked with qMLN06.157 was further confirmed its consistent expression across populations and its recessive nature of inheritance. On the basis of the consistent and effective resistance afforded by the KS23 allele at qMLN06.157, the QTL can be used in both marker-assisted forward breeding and marker-assisted backcrossing schemes to improve MLN resistance of breeding populations and key lines for eastern Africa.

Detection and Characterization of Fusarium Wilt (Fusarium oxysporum f. sp. vasinfectum) Race 4 Causing Fusarium Wilt of Cotton Seedlings in New Mexico.

Fusarium wilt (FW), caused by Fusarium oxysporum f. sp. vasinfectum (Atk.) W.C. Snyder & H.N. Hans (FOV), is one of the most destructive diseases of cotton (Gossypium spp.) worldwide. FOV race 4 (FOV4) is a highly virulent nominal race of this pathogen and a significant threat to cotton production in the western and southwestern USA and, potentially, the entire Cotton Belt. A field survey to identify FOV4 was performed in three southern counties of New Mexico in 619 cotton fields from 2018 to 2020. From 132 samples of cotton plants that exhibited wilt symptoms, Fusarium spp. were the most frequently isolated group of fungal species, with an isolation frequency of 57.4%. Eighty-four Fusarium spp. isolates were subsequently characterized by a DNA sequence analysis of three genes, EF-1α, PHO, and BT, encoding for translation elongation factor, phosphate permease, and β-tubulin, respectively. Forty-two isolates from 10 cotton fields were identified as FOV4 and confirmed with a positive 500-bp fragment diagnostic for FOV4. Twenty-six (62%) of the 42 FOV4 isolates were T type and the remainder (38%) were null type with and without a Tfo1 insertion in PHO, respectively. Each FOV4-infested field contained the same FOV4 genotype. Ten representative FOV4 isolates (one each from the 10 FOV4-infested fields) were evaluated for their pathogenicity on resistant Pima PHY 841 RF and susceptible Upland PHY 725 RF at 7, 14, 21, and 28 days after inoculation under temperature-controlled conditions at 21 to 22°C. Based on the disease severity rating, mortality rate, and area under the disease progress curve value, all 10 isolates were pathogenic to both cotton cultivars and differed in virulence; four isolates of the T genotype as a whole were more virulent than the six isolates of the N genotype. PHY 841 RF had significantly higher levels of resistance than PHY 725 RF to all FOV4 isolates. The results provide the first comprehensive account of the occurrence, distribution, and virulence of FOV4 in cotton production in New Mexico and will be useful for developing an effective strategy to manage FW in the state of New Mexico and the entire western and southwestern Cotton Belt.

Pathogenic Variability of the Jackfruit-Bronzing Bacterium Pantoea stewartii Subspecies stewartii Infection to Jackfruit Varieties and Its Pivotal Plant Hosts in Malaysia

Infection with Pantoea stewartii subspecies stewartii, which causes jackfruit-bronzing disease, is a huge problem faced by the jackfruit industry in Malaysia. This study was carried out to assess the disease ratings and aggressiveness performance of 28 Pantoea stewartii subspecies stewartii isolated from jackfruit-bronzing diseased jackfruits from four collection areas (Jenderam in Selangor State, Maran and Muadzam Shah in Pahang State, and Ipoh in Perak State) in Peninsular Malaysia, inoculated into jackfruit varieties (Tekam Yellow J33, Hong J34 and Subang Chap Boy J39), the sweetcorn variety, Mas Madu (two-week-old seedlings and nine-week-old seedlings), the cucumber variety, Rocky, and the pineapple variety, MD2. The results revealed that Pantoea stewartii subspecies stewartii produced symptoms upon all inoculations in the pathogenicity testing, thus fulfilling Koch’s postulates, except in the case of J39 and sweetcorn (two-week-old seedlings). No disease symptoms (disease rating 0) were observed in J39 and sweetcorn (two-week-old seedlings) within 14 days post-inoculation (14 dpi). The disease progression (based on disease ratings) proved that the jackfruit variety J39 was the most resistant, whereas J33 and J34 were susceptible to jackfruit-bronzing disease. The disease ratings of the 14-dpi period revealed a variability of disease progression among the 28 bacterial isolates, where the isolate JEN-14 had the fastest and highest disease ratings when inoculated into J33, J34, nine-week-old sweetcorn seedlings, cucumber, and pineapple. Likewise, the AUDPC value, based on disease rating, across the 28 isolates indicated that JEN-14 is the most aggressive and significant of the isolates (J33, J34, nine-week-old sweetcorn seedlings, cucumber, and cucumber pineapple inoculation; p &lt; 0.05). Even though isolates from Jenderam (other than JEN-14) and Maran had better disease ratings and the area under the disease progress curve (AUDPC) values than isolates from Muadzam Shah and Ipoh, no significant differences were found among the isolates (p &lt; 0.05). Based on our findings, we identified the isolate JEN-14 as the best potential candidate to assist in jackfruit-bronzing disease resistance breeding in the future. Last but not least, the methods, disease ratings, and variations of the aggressiveness profiles among the isolates from this study may be beneficial and significant by providing disease-rating references and appropriate screening approaches when selecting the most appropriately aggressive isolates for evaluating the defense response in the disease resistance breeding program.

Comparison of non-overlapping maize populations of unequal sizes for resistance to maize lethal necrosis

ABSTRACT Contrast between marker-assisted backcross (MABC) and doubled haploid (DH) methods in transferring genes for resistance to maize lethal necrosis (MLN) in maize (Zea mays L.) is not well understood. The MLN is caused by co-infection of maize plant by maize chlorotic mottle virus and sugarcane mosaic virus. Two maize panels consisting of four BC3F2 and six DH populations, separately developed through marker-assisted selection from crosses between susceptible CIMMYT lines and MLN-resistant donor parent (KS23-6), were used in the current study. The two populations were of different population structures with unequal sizes. Experiments were conducted under artificial MLN inoculations for two seasons in 2018. Analyses of variance revealed significant variations among genotypes in both panels (p ≤ 0.001). Levene’s and Welch’s tests found that variances and means of the BC3F2 and DH populations were highly unequal (p ≤ 0.001). The study identified genotypes with reduced MLN infections in both populations; however, lower means for MLN severity and area under disease progress curve (AUDPC) values, and higher heritability estimates were obtained in the DH populations than in the BC3F2 populations. Additionally, the DH populations showed higher relative genetic gains for resistance to MLN compared with the BC3F2 populations. The current study detected superiority of DH over MABC populations for breeding for resistance to MLN. Nevertheless, the results observed in the present study warrant further investigations using the same genetic materials with identical population sizes.

Fungicide Efficacy Against Pseudoperonospora humuli and Point Mutations Linked to Carboxylic Acid Amide Resistance in Michigan.

Hops have expanded as a niche crop in Michigan and other production areas in the eastern United States, but growers in these regions face annual downy mildew outbreaks incited by Pseudoperonospora humuli, exacerbated by frequent rainfall and high relative humidity. We evaluated the efficacy of foliar- and drench-applied fungicides against downy mildew and examined Michigan isolates for point mutations linked to carboxylic acid amide (CAA) resistance. Disease severity and density were assessed weekly in 2016 and 2017 in nontrellised research hop yards in Michigan. Area under the disease progress curve values for disease severity were significantly lower for plants treated with oxathiapiprolin, ametoctradin/dimethomorph, fluopicolide, cyazofamid, or mandipropamid (90.6 to 100% control) compared with those treated with fosetyl-Al (64.3 to 93.0% control) at both locations for both years. Drench treatments of fluopicolide and oxathiapiprolin/mefenoxam reduced disease density and severity at both locations but were only moderately effective (76.4 to 91.5% control). To assess CAA resistance, the cellulose synthase CesA3 gene was aligned using reference downy mildew species and primers designed to amplify the 1105 and 1109 amino acids. Point mutations conferring CAA resistance were not detected at these loci for sporangia from 42 symptomatic shoots collected from 11 commercial hop yards. These efficacy results for hop downy mildew are needed to guide disease recommendations in this expanding Michigan industry. The absence of resistant genotypes indicates that Michigan growers can continue to utilize CAA-containing commercial fungicides as part of an overall downy mildew management program.

Susceptibility of Hop Cultivars and Rootstock to Downy Mildew Caused by Pseudoperonospora humuli

Annual downy mildew [Pseudoperonospora humuli (Miyabe &amp; Takah.) G.W. Wilson, (1914)] epidemics threaten hop (Humulus lupulus L. var. lupulus) production throughout the eastern United States. Rootstock rot complicates foliar disease assessments because dormant buds may rot before producing a symptomatic basal shoot and noncolonized buds produce healthy shoots. We selected 12 cultivars to evaluate downy mildew susceptibility (2016 and 2017) and examined the rhizomes of a subset of six cultivars (2018) that showed clear differences in foliar disease ratings to determine rootstock rot susceptibility. Trials were conducted on nontrellised hop yards established at two research farms in 2016 and managed without fungicides. The relative area under the disease progress curve (rAUDPC) values for foliar disease severity and density ratings were consistently higher for ‘Cascade’, ‘Centennial’, and ‘Nugget’ than for ‘Newport’, ‘Tahoma’, and ‘Columbia’. Only ‘Centennial’ had a higher wet-rot cortex discoloration (CD) incidence (71.5%) and severity (31.9%) in its rhizomes than the other cultivars. Among cultivars with low foliar disease levels, Columbia was less vigorous (shoots/plant) than ‘Tahoma’, but had similar wet-rot CD (incidence, 46.4% to 47.2%; severity, 12.5% to 17.7%). The levels of wet-rot CD for ‘Tahoma’, ‘Newport’, and ‘Columbia’ were comparable to those of some cultivars with more severe foliar disease symptoms (‘Nugget’ and ‘Cascade’). Differences in foliar disease among cultivars with similar levels of rootstock rot suggest a resistance defense mechanism to P. humuli. Additional work is needed to clarify rootstock infection sites and characterize defense responses to aid future breeding efforts.

Induction of Systemic Resistance against Bacterial Leaf Streak Disease and Growth Promotion in Rice Plant by Streptomyces shenzhenesis TKSC3 and Streptomyces sp. SS8.

The genus Streptomyces demonstrates enormous promise in promoting plant growth and protecting plants against various pathogens. Single and consortium treatments of two selected Streptomyces strains (Streptomyces shenzhenensis TKSC3 and Streptomyces sp. SS8) were evaluated for their growth-promoting potential on rice, and biocontrol efficiency through induced systemic resistance (ISR) mediation against Xanthomonas oryzae pv. oryzicola (Xoc), the causal agent of rice bacterial leaf streak (BLS) disease. Seed bacterization by Streptomyces strains improved seed germination and vigor, relative to the untreated seed. Under greenhouse conditions, seed bacterization with consortium treatment TKSC3 + SS8 increased seed germination, root length, and dry weight by 20%, 23%, and 33%, respectively. Single and consortium Streptomyces treatments also successfully suppressed Xoc infection. The result was consistent with defense-related enzyme quantification wherein single and consortium Streptomyces treatments increased peroxidase (POX), polyphenol oxidase, phenylalanine ammonia-lyase, and β,1–3 glucanase (GLU) accumulation compared to untreated plant. Within all Streptomyces treatments, consortium treatment TKSC3 + SS8 showed the highest disease suppression efficiency (81.02%) and the lowest area under the disease progress curve value (95.79), making it the best to control BLS disease. Consortium treatment TKSC3 + SS8 induced the highest POX and GLU enzyme activities at 114.32 μmol/min/mg protein and 260.32 abs/min/mg protein, respectively, with both enzymes responsible for plant cell wall reinforcement and resistant interaction. Our results revealed that in addition to promoting plant growth, these Streptomyces strains also mediated ISR in rice plants, thereby, ensuring protection from BLS disease.

Evaluation of resistance to bacterial blight in Brazilian cassava germoplasm and disease-yield relationships

Cassava, an important source of carbohydrates in many developing countries, is affected by several diseases including bacterial blight, caused by Xanthomonas phaseoli pv. manihotis (Xpm), responsible for considerable losses in production. A 3-year split-plot design experiment was conducted to evaluate the resistance of 83 Brazilian cassava genotypes to Xpm under field conditions, inoculated or not with bacterial suspension (108 CFU mL−1). The relationship between disease progress and yield traits was also studied. Disease severity was evaluated using an ordinal 0 (no symptoms) to 8 (maximum degree of the disease) scores. At 11 months after planting, the following traits were measured: plant height, above-ground yield (measured as the total weight of leaves and stem), fresh root yield, dry matter content in the roots, and dry root yield. Based on the calculated disease index (DI) and the area under the disease progress curve (AUDPC) values, the accessions were grouped into different classes of resistance/susceptibility. There were 16 accessions classified as moderately resistant (MR), 14 as moderately susceptible (MS), 41 as susceptible (S), and 12 as extremely susceptible (ES). Complete resistance to bacterial blight was not found. Despite the significant reduction in above-ground and root yield average for the inoculated plants, when compared to non-inoculated, there was no correlation between the severity (DI) or disease progress (AUDPC) and the agronomic performance of cassava genotypes. The selection of the best 10 accessions should be a priority, considering that the agronomic performance could increase the above-ground yield, fresh root yield, and dry root yield by 22.17%, 109.61%, and 114.64%, respectively.