Susceptibility of Twenty-Three Kiwifruit Cultivars to Pseudomonas syringae pv. actinidiae

One of the best methods to control plant disease is the use of resistant cultivars. The purpose of this study is to evaluate 23 kiwifruit genotypes and cultivars for susceptibility to four strains of Psa (biovar 3) in alaboratory setting. The results showed that all the bacterial strains were pathogenic. There was no statistical difference among the bacterial strains tested. None of the kiwifruit cultivars tested were immune to Psa. There was a statistical difference in the level of susceptibility among cultivars. The cultivars Sorelli and D495/312 were the most susceptible, while the cultivar A501/44 was the most resistant. However, the above results must be verified in field conditions.

Yield losses associated with different levels of stripe rust resistance of commercial wheat cultivars in China

Wheat stripe rust is one of the most destructive diseases to affect wheat. Although the major resistant wheat varieties have made a great contribution to the global food security, yield losses due to the stripe rust still occurs in the large wheat growing areas when climatic conditions are unstable. Despite this threat, resistance levels and yield losses of these elite wheat cultivars under wheat stripe rust infection have not been well studied. Based on the present investigation of natural infection conditions over two years, analysis of the area under the disease progress curves (AUDPC) differentiated susceptible cultivars Mianmai 367 (MM367) (788.59), Jinmai 47 (JM47) (1087.71), and Avocet Susceptible (AvS) (1314.59) from resistant cultivars Xikemai 18 (XKM18) (177.50) and Xiaoyan 6 (XY6) (545.67). Stripe rust resulted in a two-year mean yield loss of 32% for all tested varieties. The susceptible varieties JM47, AvS, and MM367 lost 64%, 55%, and 21% of grain yield, respectively. On the contrary, rust-resistant cultivars XKM18 and XY6 lost only 11% and 28%, respectively. In addition, stripe rust resulted in reduced kernel hardness (KH), flour yield (FY), and flour whiteness (FW). Dough and gluten properties were also affected. Overall, results revealed that the grain yield and quality loss of the resistant wheat cultivars were less than in the susceptible cultivars. Disease-resistant cultivars such as XKM18 should be promoted and recommended for application. It may also be suggested that growing a susceptible variety such as MM367 could be feasible in combination with fungicide application under high disease pressure.

Resistant Cassava Cultivars Inhibit The Papaya Mealybug Paracoccus Marginatus Population Based On Their Interaction: From Physiological and Biochemical Perspectives

Dangerous Paracoccus marginatus papaya mealybugs cause considerable threats and challenges to cassava production and processing. The deployment of resistant cultivars offers effective, economical and eco-friendly management strategies for pest management. We utilized P. marginatus mortality, development and reproduction to evaluate the resistance of fifteen cassava cultivars and conducted physiological and biochemical analyses when P. marginatus was fed on two resistant cultivars (Myanmar, C1115) and three susceptible cultivars (BRA900, Bread, SC205). Significantly lower digestive (amylase, sucrase, lipase), detoxification (glutathione-S-transferase and carboxylesterase) and antioxidant, including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and polyphenol oxidase (PPO), enzyme activities were observed in P. marginatus feeding on resistant cultivars compared to susceptible cultivars. For resistant cultivars, a significant reduction was found in nutritional components containing free amino acids, nitrogen, soluble sugars and the secondary metabolite malondialdehyde. Additionally, significantly higher enzymatic activity (SOD, CAT, POD and PPO) levels and secondary metabolite quantities (total phenol and tannins) were found in resistant cultivars induced by P. marginatus compared with susceptible ones. Additionally, RT-qPCR tests showed that the transcripts of ten genes involved in nutrition, secondary metabolites and antioxidant activities were consistent with their physiology and biochemistry changes. Thus, resistant cultivars prevented P. marginatus populations from suffering lower P. marginatus damage by elevating secondary metabolite contents and antioxidant activities, reducing nutrition levels and decreasing enzymatic activities. This study will be beneficial in determining the important indexes for developing standard regulations to evaluate P. marginatus-resistant cassavas, helping the development of effective strategies for pest management.

Socioeconomic Challenges of California Strawberry Production and Disease Resistant Cultivars

Strawberries are the 4th highest grossing crop in California and supply 90% of US strawberries. But the industry's long reliance on the use of chemical fumigants to control soil disease, nematodes and weeds is being threatened by increased regulation of these fumigants, leading to urgent efforts to develop and test non-chemical alternatives to fumigation, such as disease resistant cultivars. Many of these technologies are promising ecologically, but making them economically viable for growers is more challenging, especially in light of the socioeconomic context of strawberry production in California that has created a state of lock-in for a sustainability transition. This paper discusses how the challenges of land prices, labor shortages, marketing standards, and low prices bear on cultivar selection. Based on qualitative interviews, we corroborate that strawberry growers operate under significant socioeconomic constraints in California, many of which are beyond their control. In addition, we find that most growers see high-yielding varieties as crucial to their economic viability with regard to land, labor, and marketing intermediaries and yet recognize that the focus on individual farm productivity works at cross purposes to the problem of poor prices. Disease resistant varieties do not at face value address the concerns voiced by most growers. Our findings suggest, however, that if some of the other pressures were exogenously mitigated, growers might be more inclined to experiment with and adopt disease resistant varieties, in combination with other approaches. The most promising policy avenues seem to therefore lie with support of grower revenues.

Genetic and Proteomic Basis of Sclerotinia Stem Rot Resistance in Indian Mustard [Brassica juncea (L.) Czern & Coss.]

Sclerotinia stem rot is one of the utmost important disease of mustard, causing considerable losses in seed yield and oil quality. The study of the genetic and proteomic basis of resistance to this disease is imperative for its effective utilization in developing resistant cultivars. Therefore, the genetic pattern of Sclerotinia stem rot resistance in Indian mustard was studied using six generations (P1, P2, F1, F2, BC1P1, and BC1P2) developed from the crossing of one resistant (RH 1222-28) and two susceptible (EC 766300 and EC 766123) genotypes. Genetic analysis revealed that resistance was governed by duplicate epistasis. Comparative proteome analysis of resistant and susceptible genotypes indicated that peptidyl-prolyl cis-trans isomerase (A0A078IDN6 PPIase) showed high expression in resistant genotype at the early infection stage while its expression was delayed in susceptible genotypes. This study provides important insight to mustard breeders for designing effective breeding programs to develop resistant cultivars against this devastating disease.

Impact of Host Resistance to Tomato Spotted Wilt Orthotospovirus in Peanut Cultivars on Virus Population Genetics and Thrips Fitness

Thrips-transmitted tomato spotted wilt orthotospovirus (TSWV) is a major constraint to peanut production in the southeastern United States. Peanut cultivars with resistance to TSWV have been widely used for over twenty years. Intensive usage of resistant cultivars has raised concerns about possible selection pressure against TSWV and a likelihood of resistance breakdown. Population genetics of TSWV isolates collected from cultivars with varying levels of TSWV resistance was investigated using five TSWV genes. Phylogenetic trees of genes did not indicate host resistance-based clustering of TSWV isolates. Genetic variation in TSWV isolates and neutrality tests suggested recent population expansion. Mutation and purifying selection seem to be the major forces driving TSWV evolution. Positive selection was found in N and RdRp genes but was not influenced by TSWV resistance. Population differentiation occurred between isolates collected from 1998 and 2010 and from 2016 to 2019 but not between isolates from susceptible and resistant cultivars. Evaluated TSWV-resistant cultivars differed, albeit not substantially, in their susceptibility to thrips. Thrips oviposition was reduced, and development was delayed in some cultivars. Overall, no evidence was found to support exertion of selection pressure on TSWV by host resistance in peanut cultivars, and some cultivars differentially affected thrips fitness than others.

A Novel QTL for Resistance to Phytophthora Crown Rot in Squash

Phytophthora capsici Leonian causes significant yield losses in commercial squash (Cucurbita pepo) production worldwide. The deployment of resistant cultivars can complement integrated management practices for P. capsici, but resistant cultivars are currently unavailable for growers. Moderate resistance to Phytophthora crown rot in a selection of accession PI 181761 (C. pepo) (designated line #181761-36P) is controlled by three dominant genes (R4, R5 and R6). Introgression of these loci into elite germplasm through marker-assisted selection (MAS) can accelerate the release of new C. pepo cultivars resistant to crown rot, but these tools are currently unavailable. Here we describe the identification of a quantitative trait locus (QTL), molecular markers and candidate genes associated with crown rot resistance in #181761-36P. Five hundred and twenty-three SNP markers were genotyped in an F2 (n = 83) population derived from a cross between #181761-36P (R) and Table Queen (S) using targeted genotyping by sequencing. A linkage map (2068.96 cM) consisting of twenty-one linkage groups and an average density of 8.1 markers/cM was developed for the F2 population. The F2:3 families were phenotyped in the greenhouse with a virulent strain of P. capsica, using the spore-spray method. A single QTL (QtlPC-C13) was consistently detected on LG 13 (chromosome 13) across three experiments and explained 17.92–21.47% of phenotypic variation observed in the population. Nine candidate disease resistance gene homologs were found within the confidence interval of QtlPC-C13. Single nucleotide polymorphism (SNP) markers within these genes were converted into Kompetitive Allele Specific PCR (KASP) assays and tested for association with resistance in the F2 population. One SNP marker (C002686) was significantly associated with resistance to crown rot in the F2 population (p < 0.05). This marker is a potential target for MAS for crown rot resistance in C. pepo.

Screening of Early Maturing Soybean Accessions for Resistance against HG Type 2.5.7 of Soybean Cyst Nematode, Heterodera glycines

Soybean cyst nematode (SCN; Heterodera glycines) is one of the devastating soybean pests worldwide, including the United States. Resistant cultivars combined with crop rotation are the primary methods for managing this nematode. SCN is known to have genetically diverse populations and can develop new virulent forms over time due to the continuous planting of cultivars derived from same source of resistance. Thus, identifying novel SCN resistant sources is of paramount importance for soybean breeding for nematode resistance. In this study, we screened 149 early maturity soybean [Glycine max (L.)] accessions for resistance to SCN HG type 2.5.7, which is one of the prevalent virulent SCN populations in North Dakota. SCN white females were extracted from individual plants of each accession after 35 days of growth in greenhouse conditions. The females were counted to determine a female index [FI = (average number of females on a tested accession/average number of females in Barnes, a susceptible soybean check) x 100]. The resistance response of each soybean accession was categorized as resistant, moderately resistant, moderately susceptible, and susceptible. Out of the soybean 149 accessions tested, only 13 were resistant in both runs of the experiments. The majority of screened soybean accessions were susceptible or moderately susceptible to the SCN HG type 2.5.7. The resistant soybean accessions identified in this study have the potential to be used in breeding SCN-resistant cultivars after further elucidation of the resistance genes or loci.

Virulence Characterization of Puccinia graminis f. sp. avenae and Resistance of Oat Cultivars in China

Oat stem rust, caused by Puccinia graminis f. sp. avenae (Pga), is one of the most devastating diseases of oat. The most cost-effective and eco-friendly strategy to control this disease is the use of resistant cultivars. However, P. graminis f. sp. avenae can overcome the resistance of cultivars by rapidly changing its virulence. Thus, information on the virulence of P. graminis f. sp. avenae populations and resistance of cultivars is critical to control the disease. The current study was conducted to monitor the virulence composition and dynamics in the P. graminis f. sp. avenae population in China and to evaluate resistance of oat cultivars. Oat leaves naturally infected by P. graminis f. sp. avenae were collected during 2018 and 2019 and 159 isolates were derived from single uredinia. The isolates were tested on 12 international differential lines, and eight races, TJJ, TBD, TJB, TJD, TJL, TJN, TGD, and TKN, were identified for the first time in China. The predominant race was TJD, virulent against Pg1, Pg2, Pg3, Pg4, Pg8, Pg9, and Pg15, accounting for 35.8% and 37.8% in 2018 and 2019, respectively. The sub-predominant races were TJN (30.2% in 2018, 28.3% in 2019) and TKN (20.8% in 2018, 12.3% in 2019). All isolates were virulent to Pg1, Pg2, Pg3, and Pg4, and avirulent to Pg6 and Pg16. The three predominant races (TJD, TJN, and TKN) were used to evaluate resistance in 30 Chinese oat cultivars at the seedling and adult-plant stages. Five cultivars, Bayan 1, Baiyan 2, Baiyan 3, Baiyan 5, and Baiyan 9, were highly resistant to the three races at both seedling and adult-plant stages. The results of the virulences and frequencies of P. graminis f. sp. avenae races and the resistant cultivars will be useful in understanding the pathogen migration and evolution and for breeding oat cultivars with stem rust resistance.