Changes in Phenolics, Polyphenol Oxidase and its Isoenzyme Patterns in Relation to Resistance in Taro against Phytophthora colocasiae

The effect of Phytophthora leaf blight disease, caused by Phytophthora colocasiae Raciborski, on the accumulation of phenolics and polyphenol oxidase (PPO) activity in ex vitro plants was studied in three resistant (DP‐25, Duradim and Jhankri) and one susceptible (N‐118) genotypes of taro [Colocasia esculenta (L). Schott]. The inoculation of taro leaves with P. colocasiae spores resulted in a quantitative change in both biochemical parameters and induction of PPO isoforms in resistant genotypes. The amount of phenolics was increased owing to blight by 68.02%, 58.87%, 52.67% and 11.50% in DP‐25, Duradim, Jhankri and N‐118, respectively. The per cent increase in PPO under stress over non‐stress condition was also highest in DP‐25 (49.14%) followed by Duradim (41.56%), Jhankri (40.55%) and N‐118 (17.08%). The resistant genotypes showed higher activity of PPO as compared with susceptible ones, which was reflected through its banding pattern in isoenzyme analysis, detecting four different isoforms. The intensity of the bands was higher in the resistant genotypes than in susceptible N‐118. The appearance of high intensity bands and/or reduction in the intensity of particular isoform(s) in the zymograms of all the three resistant taro genotypes studied, led to the apparent conclusion of linking PPO isoenzyme expression with blight resistance in taro. The blight incidence (per cent leaf infection and leaf area infection) was lower in the resistant genotypes than in susceptible, N‐118. The yield reduction owing to blight was below 20% in DP‐25, Jhankri and Duradim, while the same was more than 40% in N‐118. The phenolics and PPO activity was negatively correlated with disease incidence and yield reduction owing to blight. Based on the results of disease incidence, biochemical contents and yield, the pattern of stress tolerance was DP‐25 > Duradim > Jhankri > N‐118. The studied parameters, i.e. phenolics and PPO could be used as biochemical markers for leaf blight stress tolerance studies in taro.     

Antioxidative enzymes and isozymes analysis of taro genotypes and their implications in <Emphasis Type="Italic">Phytophthora</Emphasis> blight disease resistance

Assessment of the differential expression of antioxidative enzymes and their isozymes, was done in 30 day-old ex vitro raised plants of three highly resistant (DP-25, Jhankri and Duradim) and one highly susceptible (N-118) genotypes of taro [Colocasia esculenta (L.) Schott]. Antioxidative enzymes were assayed in the ex vitro plants, 7 days after inoculation with the spores (15,000 spores ml⁻¹ water) of Phytophthora colocasiae Raciborski to induce taro leaf blight disease. Uninoculated ex vitro plants in each genotype were used as control. The activity of superoxide dismutase (SOD) and guaiacol peroxidase (GPX) increased under induced blight condition when compared with control. Increase in antioxidative enzymes was more (67–92%) in the resistant genotypes than that (21–29%) of the susceptible genotype. The zymograms of SOD and GPX in the resistant genotypes, with pathogenic infection, showed increased activity for anodal isoform of SOD and increased expression and/or induction of either POX 1 or POX 2 isoforms of GPX. In susceptible genotype, expression of the above isoforms was faint for SOD and nearly absent for GPX under both blight free and induced blight conditions. Induction and/or increased activity of particular isoform of SOD and GPX against infection of Phytophthora colocasiae in the resistant genotypes studied led to the apparent conclusion of linkage of isozyme expression with blight resistance in taro. This might be an important criterion in breeding of taro for Phytophthora leaf blight resistance.     

A Rapid and Efficient Method for In Vitro Screening of Taro for Leaf Blight Disease Caused by Phytophthora colocasiae

Taro leaf blight caused by Phytophthora colocasiae presents the single biggest constraint for taro cultivation globally. To accelerate breeding and selection for disease resistance to leaf blight, it is important to develop bioassays which could differentiate resistant and susceptible cultivars efficiently. In this study, thirty taro accessions and four released cultivars were evaluated for resistance to leaf blight using a modified floating leaf disc assay. A novel method for mass production of P. colocasiae zoospores was developed and used as inoculum for the assay. There were significant (P < 0.05) differences among accessions in their response to P. colocasiae infection in the detached leaf assay. The accessions could be efficiently classified into various resistance groups based on a 0–4 score. Also, the assay results were consistent with the field evaluation scores of taro accessions. Thus, this study reports the development of a rapid, simple and repeatable assay that can be used to screen large numbers of taro cultivars for resistance to P. colocasiae.     

Morphophysiological and biochemical attributes influence intra-genotypic preference of shoot fly [Atherigona soccata (Rondani)] among sorghum genotypes

Shoot fly [Atherigona soccata (Rondani)] is a destructive pest of sorghum at the seedling stage and causes huge losses to grain yield and green fodder. The host-plant resistance mechanism is the best approach to reduce the attack of insects in plants. The damage parameters, morphophysiological traits, and biochemical metabolites had been investigated in the leaves and stem of contrasting sorghum genotypes, viz., resistant (IS18551, ICSV705, ICSV700), moderately resistant (PSC-4), and susceptible (SWARNA and SL-44) at 15 and 21 days after emergence (DAE) against shoot fly infestation. The resistant genotypes recorded lowest shoot fly oviposition and incidence (0.3–0.7 eggs plant⁻¹ and 10–15%) than the susceptible genotypes (2.4–3.0 eggs plant⁻¹ and 70–80%), respectively. The susceptible genotype SWARNA recorded 50% and 80% higher deadheart formation than the resistant genotype IS18551 at 15 and 21 DAE, respectively. Resistant genotypes exhibited higher trichome density at adaxial and abaxial part of leaf (118–145 and 106–131) with pink colored leaf sheath (scale 1.50–3.25), glossy leaves (scale1.00–1.25), and lower leaf surface wetness (scale1.25–2.00) compared with susceptible genotype with 49.3–73.3 and 25.3–64.0, scale 2.50–4.00, scale 2.75–3.50, and scale 3.25–4.25 for the respective parameters. Another defense response of sorghum toward the insect attack was modulation of plant metabolism. The infested genotypes responded to insect attack by upregulation of total soluble sugar, total phenol, prussic acid, and chlorophyll content by 1.2–2.1-fold, 1.5–2.0-fold, 1.2–1.3-fold, and 1.2–3.9-fold with more induction in susceptible genotypes at 21 DAE. On the whole, the present study indicates that morphophysiological and biochemical attributes contribute toward the resistance mechanism in sorghum against shoot fly infestation.

     

Isolation of anti-fungal phenolic compounds from petioles of two Hevea brasiliensis (rubber) genotypes and their effect on Phytophthora meadii

Phenolic compounds were present in greater amounts in non‐infected petioles of genotypes of Hevea brasiliensis that are resistant to Phytophthora leaf disease than in genotypes that are susceptible. Phenolic compounds extracted from petioles of either susceptible (PB86) or resistant (RRIC100) genotypes, before or after infection with Phytophthora meadii, had anti‐fungal properties. Artificially infected petioles of PB86 had phenolic acids, triterpenoids or flavonoids, whereas healthy petioles contained only triterpenoids or flavonoids. However, healthy or infected petioles of RRIC100 contained only trace amounts of the above compounds and of vanillin (3‐methoxy‐4‐hydroxybenzaldehyde). Vanillin and umbelliferone (7‐hydroxycoumarin) were shown to suppress zoospore germination of P. meadii on glass slides and to inhibit its growth in pea broth and V‐8 juice agar. Vanillin was slightly more active than umbelliferone. Resistance of RRIC100 to Phytophthora was suspected as being related to the polymerisation of phenolic compounds to form lignin, which may suppress further spread of the pathogen's mycelium into healthy tissues. Formation of lignin from phenolic aldehydes as a barrier to disease spread may be a critical factor in resistance.     

Correlative evidence for peroxidase involvement in disease resistance against <Emphasis Type="Italic">Alternaria</Emphasis> leaf blight of tomato

Eighteen tomato genotypes, with varying degree of response to Alternaria leaf blight disease (ALBD) were used to assess the possible involvement of protease and peroxidase activities in disease response. Pre-infectional protease activity varied noticeably in tested genotypes. Highest pre-infectional protease activity was observed in susceptible genotype CLN-2123. Post-infectional protease activity level was generally lower when compared with pre-infectional level in all genotypes with exception of unchanged level in Tibrido. There was no correlation between post-infectional protease activity and percent disease index (%DI). In contrast, pre- and post-infectional leaf peroxidase activities showed a significant (p < 0.01) negative correlation with %DI. Genotypes with higher pre-infectional peroxidase activity performed better on exposure to Alternaria alternata infection and accumulate enhanced peroxidase activity. Tibrido accumulated highest peroxidase activity while level was lowest in 1621P, which showed highest ALBD incidence. Moreover, genotypes with better resistance to A. alternata infection maintained higher post-infectional peroxidase activity. In resistant (Tibrido) and all moderately resistant genotypes, leaf peroxidase activity raised after inoculation when compared with the pre-inoculation level. I summary, higher pre- and post-infectional peroxidase activity was found to be associated with Alternaria leaf blight resistance. The peroxidase activity can be used as a biochemical tool in marker-assisted screening of tomato germplasm for Alternaria leaf blight resistance.     

Assessment of genetic diversity among soybean genotypes differing in response to aerial blight (Rhizoctonia solani Kuhn) using SSR markers

Forty‐seven genotypes and one wild relative of soybean, Glycine soja, were screened for resistance against aerial blight under epiphytotic conditions in the field during the Kharif season of two consecutive years viz., 2016 and 2017. Out of the 48 genotypes screened, only 18 genotypes exhibited a moderately resistant response to aerial blight during both the years of study. In order to perform molecular screening of the genotypes for aerial blight resistance, the genomic DNA obtained from the seedlings of the forty‐eight soybean genotypes was subjected to PCR amplification with 12 SSR markers. The SSR markers Satt 119, Sat_076, Satt 433, Satt 281, Satt 277, Satt 245 and Satt 520 were able to clearly amplify different banding pattern for resistant and susceptible genotypes, out of which Satt 433 and Satt 520 were found to exhibit a pattern, highly similar to the results of field screening of the genotypes with respect to resistant and susceptible reaction to the disease. The eighteen soybean genotypes that exhibited moderately resistant reaction to RAB under field conditions during both the years showed a banding pattern similar to resistant check PS‐1583 in the amplification profile produced by the SSR markers. The polymorphism information content (PIC) from the analysis of amplification profile of the SSR markers used in the study, ranged from 0.58 to 0.95. The dendrogram constructed using UPGMA cluster analysis clearly differentiated the resistant and susceptible genotypes of soybean into two separate groups.     

Use of a Mini-Dome Bioassay and Grafting to Study Resistance of Chickpea to Ascochyta Blight

A mini‐dome bioassay was developed to study pathogenicity of Ascochyta rabiei and relative resistance of chickpea (Cicer arietanium). It was determined that the best condition for assaying pathogenicity of A. rabiei was to use 2 × 10⁵ spores/ml as inoculum and to maintain a leaf wetness period of 24 h under mini‐domes at a temperature between 16 and 22°C. This mini‐dome pathogenicity assay was used to determine relative resistance of six chickpea cultivars (cvs) to isolates of two pathotypes of A. rabiei. Grafting was employed to detect any translocated factors produced in the chickpea plant that mediate disease response, which could help elucidate possible resistance mechanisms to Ascochyta blight. The six chickpea cv. were grafted in all possible scion–rootstock combinations, and then inoculated with isolates of two pathotypes of A. rabiei using the mini‐dome technique. Results showed that self‐grafted‐resistant plants remained resistant and self‐grafted‐susceptible plants stayed susceptible, indicating the grafting procedure did not alter host response to infection by A. rabiei. Susceptible scions always exhibited high and similar levels of disease severity regardless of rootstock genotypes, and resistant scions always showed low and similar levels of disease severity when they were grafted onto any of the six rootstock genotypes. Orthogonal contrasts showed that scion genotypes determined disease phenotype, and that rootstock genotypes had no contribution to disease phenotype of the scions. The pathogenicity assay did not detect any translocated disease‐mediating agents responsible for susceptibility or resistance in chickpea. Disease phenotypes of Ascochyta blight of chickpea were conditioned locally by scion genotypes.     

Evaluation of fungicides on Indian isolates of Phytophthora colocasiae causing leaf blight of taro

Leaf blight caused by Phytophthora colocasiae is the most destructive disease affecting taro (Colocasia esculenta) worldwide including India. Fungicides (primarily metalaxyl) remain as an important strategy to manage taro leaf blight in India over decades. It is important to monitor isolate sensitivity to identify build-up of fungicide resistance and thereby modify fungicide usage strategies. P. colocasiae isolates representing four different geographical regions of India were evaluated for their sensitivity to metalaxyl and three other commercially available fungicides viz. Samarth, Biofight and Akoton by poisoned media technique. All the isolates tested were sensitive to metalaxyl, nevertheless there is an increase in the effective concentration compared to the previous reports. Among the other fungicides, Samarth was found to be superior in completely inhibiting mycelial growth at 0.05% followed by Biofight at 1%. Metalaxyl and Akoton^® shared a common inhibitory concentration at 2%. The most effective fungicide determined by the in vitro method was evaluated in vivo for studying the pattern of inhibition before and after the disease development in detached taro leaf. The results of the study revealed that build-up on metalaxyl resistance in P. colocasiae is in its course and caution should be taken while administering against taro leaf blight. Fungicide Samarth could be used as an alternative to metalaxyl for management of taro leaf blight.     

重庆加工型辣椒种质资源抗疫病鉴定的分子标记筛选

为筛选出适用于重庆加工型辣椒疫病抗性鉴定的分子标记,以63份重庆加工型辣椒种质资源为材料,研究了12个辣椒疫病抗性相关分子标记的筛选效率。结果表明,7个分子标记在种质间无差异条带,2个分子标记的扩增结果不稳定,只有ZL6203、ZL6726和E73等3个分子标记在种质间能扩增出差异条带。ZL6203筛选高抗、抗性和中抗材料的效率分别为87.50%、77.78%和63.64%;ZL6726筛选抗性和感病材料的效率分别为100.00%和66.67%;E73筛选抗性和高抗材料的效率分别为77.78%和62.50%。因此,同一辣椒抗疫病分子标记对不同抗性材料筛选效率存在较大差异,ZL6203、ZL6726和E73适用于重庆地区加工型辣椒抗疫病材料的鉴定。     

The effect of leaf exudates on the spore germination of phylloplane mycoflora of chilli (Capsicum annuum L.) cultivars

Summary Ninetten aminoacids, twelve sugars, eleven organic acids and ten phenols were detected in the leaf exudates of three cultivars of chilli. The number of aminoacids, sugars, organic acids and phenols increased as the plants grew older. More aminoacids and sugars were detected in the exudate from the susceptible cultivar (Malwa). More organic acids and phenols were detected from the resistant cultivar (Simla). The leaf exudate of the resistant cultivar (Simla) inhibited spore germination of the pathogen (Alternaria solani) while that of susceptible (Malwa) stimulated spore germination. The cultivar ‘Patna’ which is moderately resistant, occupied an intermediate position. Spore germination of the isolated fungi was enhanced in leaf exudate of susceptible cultivar (Malwa), while leaf exudates of the moderately resistant (Patna) and resistant (Simla) inhibited spore germination of the majority of fungi isolated. Most of the antagonistic fungi were not isolated from the susceptible cultivar and the percentage spore germination of these fungi was less in leaf exudate of the susceptible cultivar, while leaf exudates of resistant cultivars enhanced the percentage spore germination of antagonistic fungi,viz Aspergillus flavus, A. fumigatus, A. versicolor, Penicillium citrinum, P. restrictum andTrichoderma viride.     

The genetic structure of taro: a comparison of RAPD and isozyme markers

Germplasm characterization and evolutionary process in viable populations are important links between the conservation and utilization of plant genetic resources. Here, an investigation is made, based on molecular and biochemical techniques for assessing and exploiting the genetic variability in germplasm characterization of taro, which would be useful in plant breeding and ex situ conservation of taro plant genetic resources. Geographical differentiation and phylogenetic relationships of Indian taro, Colocasia esculenta (L.) Schott, were analyzed by random amplified polymorphic DNA (RAPD) and isozyme of seven enzyme systems with specific reference to the Muktakeshi accession, which has been to be proved resistant to taro leaf blight caused by P. colocasiae. The significant differentiations in Indian taro cultivars were clearly demonstrated by RAPD and isozyme analysis. RAPD markers showed higher values for genetic differentiation among taro cultivars and lower coefficient of variation than those obtained from isozymes. Genetic differentiation was evident in the taro accessions collected from different regions of India. It appears that when taro cultivation was introduced to a new area, only a small fraction of genetic variability in heterogeneous taro populations was transferred, possibly causing random differentiation among locally adapted taro populations. The selected primers will be useful for future genetic analysis and provide taro breeders with a genetic basis for selection of parents for crop improvement. Polymorphic markers identified in the DNA fingerprinting study will be useful for screening a segregating population, which is being generated in our laboratory aimed at developing a taro genetic linkage map.     

Rapid screenhouse assessment of bacterial blight in cassava genotypes in Nigeria

Bacterial blight disease caused by Xanthomonas axonopodis pv. manihotis (Berthet-Bondar) Dye was assessed in 11 artificially inoculated cassava genotypes in a screenhouse. Disease progress was estimated at intervals of 3 days by measuring the length of necrotic lesions on stems and leaves, as well as estimating the average disease score and area under disease progress curve (AUDPC). Based on the average disease scores, cassava genotypes 30572, TME 1, TME 7 and TME 9 were classified as resistant to bacterial blight, genotypes 4(2)1425, TME 2, TME 4 and TME 12 were tolerant while cassava genotypes 30001, TME 3, and TME 28 were susceptible. Direct correlations, statistically significant at p < 0.05, were obtained between stem necrosis, leaf necrosis, average disease scores and AUDPC in the 11 cassava genotypes. Screenhouse experiments afford rapid assessment of resistance status of cassava genotypes to bacterial blight in Nigeria.     

Fusarium Head Blight Reactions and Accumulation of Deoxynivalenol (DON) and Some of its Derivatives in Kernels of Wheat, Triticale and Rye

Fifty-three commercially grown cultivars and germplasm lines of winter triticale (n = 18), wheat (n = 13), and rye (n = 5) and spring triticale (n = 8), wheat (n = 7) and rye (n = 2) were inoculated at mid anthesis with a spore suspension consisting of a mixture of Fusarium culmorum, Fusarium avenaceum and Fusarium graminearum isolates of known toxinogenic activity. Reactions to Fusarium head blight were measured as disease severity, reductions of kernel number/head, kernel weight/head and 1000 kernel weight, number of Fusarium-damaged kernels and kernel content of deoxynivalenol (DON) and its acetyl-derivatives 3-AcDON, 15-AcDON, and moniliformin. None of the cereal genotypes was completely resistant to Fusarium head blight. Wheat suffered from the largest kernel weight reductions, and accumulated the largest amounts of deoxynivalenol (up to 39.5 mg/kg) and 3AcDON (up to 6.0 mg/kg) in kernels. Deoxynivalenol was not detected in grain samples of winter rye cv. Dańkowskie Z?ote, and spring rye cv. Ludowe. 15-AcDON was only detected in genotypes of triticale, and 3AcDON only in a few genotypes of winter wheat and rye. Moniliformin was detected at low concentrations (up to 0.092 mg/kg) in kernels of some genotypes selected for the mycotoxin analysis. A moderately strong Pearson correlation was found between head blight severity parameters and the accumulation of deoxynivalenol and its derivatives in grain of the cereal genotypes studied. Fusarium head blight severity parameters were correlated with the percentage of Fusarium-damaged kernels and reductions of yield components. However, some head blight-susceptible genotypes realized their potential yields, but accumulated high levels of mycotoxins in kernels. Both Fusarium head blight resistant and susceptible genotypes of the three cereal species accumulated deoxynivalenol in kernels. This finding suggests that the system regulating deoxynivalenol accumulation may be independent of Fusarium head blight reaction.     

Isozymes variability among <Emphasis Type="Italic">Fusarium udum</Emphasis> resistant cultivars of pigeonpea (<Emphasis Type="Italic">Cajanus cajan</Emphasis> (L.) (Millsp)

The present study was carried out to select the different pigeonpea cultivars for resistance against wilt caused by Fusarium udum and to assess the genetic variability among the resistant and susceptible cultivars. These cultivars were screened by root dip inoculation and classified into resistant (ICP 8863 and 9145), moderately resistant (ICP 11681 and Selection-1), susceptible (ICP 7118, TRG-1 and LRG-30) and highly susceptible cultivars (ICP-2376 and LRG-41). The peroxidase activity (PEO) in both leaf and root tissues of four pigeonpea cultivars (ICP 8863, Selection-1, ICP 2376 and LRG-30) were determined at 1^st, 4^th and 7^th day after inoculation (DAI) in healthy and F. udum infected tissues. Higher PEO activity in both leaf and root was observed and at 4^th DAI in susceptible cultivars. In native-PAGE analysis of isozymes, the induction of specific leaf peroxidase band (Em=0.17) and two root peroxidase bands (Em=0.24 and 0.55) were observed in ICP 8863 after inoculation. Significant differences were observed in the leaf phosphatase and esterase banding profiles of all the cultivars. The presence of leaf phosphatase band at Em of 0.04 was observed only in ICP 8863 and 11681. The leaf esterase band (Em=0.3) was well expressed in ICP 8863 when compared to other cultivars. The significance of peroxidase in plant defense mechanism and utility of biochemical markers in breeding programmes are discussed. Part of M.Sc. (Ag) thesis of the first author and approved by the Acharya N.G. Ranga Agricultural University during March 2002.     

Carbohydrate, Amino acid, Phenolic and Mineral Nutrient Contents of Pepper Plants in Relation to Age-Related Resistance to Phytophthora capsici

The relationship between age-related resistance of peper plants to Phytophthora capsici and contents of carbohydrates, amino acids, phenolics and mineral nutrients in pepper stems was studied using two pepper cultivars, Hanbyul (susceptible) and Kingkun (resistant). With increasing age of pepper plants, the two cultivars, which differ in their susceptibility to Phytophthora blight, became gradually resistant to the disease. The cultivar Kingkun distinctly showed the age-related resistance to Phytophthora blight at the second branch stage. The weight of dry matter in healthy stems of pepper plants at the second branch stage was twice that at the six leaf stage. The resistant cultivar Kingkun contained lower levels of fructose, glucose and sucrose in stems than the susceptible cultivar Hanbyul at the different developmental stages. No consistent differences between the developmental stages of the plants were recognized with regard to their glucose content. However, the contents of fructose and sucrose in the cultivar Hanbyul greatly increased at the second branch stage. The levels of inositol reduced in both pepper cultivars during plant development. In view of the fact that there were only slight changes in the amount of total amino acids, it seems unlikely that there is a relationship between the amino acid metabolism and the retardation of Phytophthora infection during plant development. The amounts of total phenolic compounds in pepper stems were relatively low at the later growth stages of the plants and also in the resistant cultivar Kingkun. The contents of macroelemental nutrients such as nitrogen, phosphorus, potassium, calcium and magnesium were drastically reduced in pepper stems at the later plant growth stage. No significant differences between the cultivars or the plant growth stages were found in the silicon and microelemental nutrients such as sodium, iron, zinc and manganese. These results suggest that the expression of age-related resistance of pepper plants may be due to the morphological and nutritional changes in tissues of pepper stems during ageing, i.e. the pronounced increase in weight of dry matter, the significant decrease in amounts of mineral nutrients such as nitrogen, phosphorus, potassium, calcium and magnesium, and the tow contents of fructose, glucose and sucrose in the stem tissues.     

Screening Study of Potential Lead Compounds for Natural Product-based Fungicides Against Phytophthora Species

Phytophthora spp. is one of the phytopathogenic Oomycete responsible for many important crop losses. Relevant species are P. infestans (causing potato late blight) and P. capsici (causing blight in pepper). In recent years, the use of conventional fungicides has favoured the appearance of different resistant strains. This study analyses the effect of various compounds on these two Phytophthora species. Those compounds were designed on the basis of known structures of natural compounds to obtain a rational control of these fungal‐like species. All the analysed products showed a fungistatic activity against both strains, one of them reduced mycelial growth by over 46% at 100 p.p.m.     

Identification of phloem sieve elements as the site of resistance to silverleaf whitefly in resistant alfalfa genotypes

Experiments were conducted to locate the plant tissue where resistance is expressed against silverleaf whitefly, Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae), in alfalfa, Medicago sativa L. (Fabaceae), genotypes previously shown to have high levels of resistance against this pest. Previous work demonstrated that resistance in the resistant alfalfa genotypes was expressed primarily as high first‐instar mortality; consequently this study focused on first‐instar nymphs. Examination of stylets in cleared leaf tissue indicated that first‐instar nymphs located vascular bundles with equal success on resistant and susceptible alfalfa genotypes. Furthermore, direct current electrical penetration graphs (DC‐EPG) indicated that sieve elements were penetrated and phloem ingestion behavior was initiated with equal success on resistant and susceptible genotypes. Thus, the mechanism of resistance does not reside in tissues encountered by the stylets prior to penetrating a phloem sieve element. Honeydew production (as a proxy for ingestion) was greatly reduced on two resistant genotypes compared to the two susceptible genotypes. The frequency distribution of honeydew production was bimodal, indicating that most individuals on the resistant genotypes produced little or no honeydew while some produced as much honeydew as whiteflies on the susceptible genotypes. This indicates that expression of resistance is an all‐or‐nothing phenomenon; an individual nymph either encounters resistance and cannot sustain ingestion or it does not encounter resistance and ingests just as well as on a susceptible plant. Intermediates are rare. DC‐EPGs indicate that phloem ingestion behavior is significantly reduced on two of the resistant genotypes compared to the susceptible genotypes. The primary reason for this appears to be more frequent termination of phloem ingestion behavior on at least one of the resistant genotypes. On one of the resistant genotypes, the productivity of EPG‐measured phloem ingestion behavior (honeydew produced per min of phloem ingestion behavior) was reduced compared to a susceptible control.     

Components of Resistance to Early Blight in Four Potato Cultivars: Effect of Leaf Position

Components of early blight resistance were quantified in leaves of different ages in four potato cultivars. The components of resistance: incubation period (IP), lesion number (LN), early blight severity, lesion expansion rate (LER), latent period (LP) and spore production by lesion area (SPLA), were evaluated separately in the lower, middle and upper leaves of four potato cultivars. Plants of cultivar Aracy (resistant), Delta (moderately resistant), Desirée (susceptible) and Bintje (susceptible) were inoculated with an Alternaria solani isolate at the beginning of the flowering stage. Disease severity varied in different plant parts. In all cultivars, regardless of resistance, the smallest values of LN, and severity were recorded on the upper leaves, suggesting that young tissues are less susceptible. In cultivar Aracy, the IP was long, with small values of LN and LER and consequently, low values of early blight severity in all leaf positions were recorded. Although IP was long in cultivar Aracy, no differences between the moderately resistant cultivar Delta and the susceptible cultivars Bintje and Desirée could be detected for this component. The IP was only influenced by leaf position in cultivar Aracy. Clear differences in resistance levels among cultivars could be detected regarding LN, severity and LER. However, neither LP nor SPLA were associated with resistance level of cultivars or with leaf position. Analyses according to plant part suggest that evaluations on leaves of the middle third part are most suitable for screening for early blight resistance in potato.