Factors affecting the field infection of potato tubers of different cultivars by blight (Phytophthora infestans)

The relationships between rain and blight (Phytophthora infestans) were studied in unsprayed crops of cultivars differing widely in foliage and tuber susceptibility. The occasions when tubers were infected depended on rain and not cultivar, but numbers of tubers infected after rain was affected by the blight susceptibility of the cultivar. Infected tubers were first found when less than 5 % (BMS key) of the potato foliage was infected but few fresh infections occurred when 50–75% of the foliage had been destroyed. Some tubers were infected after 8 mm rain (tubers near the surface with even less) but large increases in numbers of tubers infected usually occurred only after 25 mm or more had increased soil moisture to above ‘field capacity’ around the tuber for at least 24 h. The most susceptible cultivars Ulster Ensign and Arran Banner had all plants with some tuber blight, and some plants with all tubers affected and often many lesions per tuber. Cultivars of intermediate susceptibility, King Edward and Up-to-Date, had some plants without blighted tubers, many with a few and very few with all. The more resistant cultivars Majestic and Arran Viking had many plants without infected tubers and many lesions that aborted while still necrotic threads, so that the fungus did not spread. Most infections occurred through tuber eyes, lenticels or sometimes growth cracks. The distribution of blight lesions on tubers differed in the different seasons, for example, lenticels were most commonly infected on Arran Banner and Ulster Ensign and eyes on King Edward, Majestic and Arran Viking. In late or slowly developing attacks, lesions on stems became more numerous and larger than in fast, early attacks and were prolific sources of spores on King Edward and Up-to-Date but not on Majestic and Arran Viking. Because much rain water runs down the stems of Up-to-Date and King Edward, stem lesions can provide an important source of inoculum for tubers.     

Relationship between Symptom Development and Actual Sites of Infection in Leaves of Anthurium Inoculated with a Bioluminescent Strain of Xanthomonas campestris pv. dieffenbachiae

The infection process of bacterial blight of anthurium was monitored with a bioluminescent strain of Xanthomonas campestris pv. dieffenbachiae. The relationship between symptom expression on infected leaves (assessed visually) and the extent of bacterial movement within tissues (evaluated by bioluminescence emission) varied among anthurium cultivars. In several cultivars previously considered susceptible on the basis of symptom development alone, bacterial invasion of leaves extended far beyond the visually affected areas. In other cultivars previously considered resistant, bacterial invasion was restricted to areas with visible symptoms. In three cultivars previously considered resistant, leaves were extensively invaded by the bacterium, and yet few or no symptoms were seen on infected leaves. The pathogen was consistently recovered from leaf sections emitting bioluminescence but not from sections emitting no light. At an early stage of infection, no significant differences in the percentages of infected areas as determined by visual assessment were observed in any of the cultivars. However, differences among cultivars were detected by bioluminescence as the disease progressed, because bacterial invasion was not always accompanied by symptom expression. In susceptible cultivars, the advancing border of infection was 5 to 10 cm inward from the margins of the visible symptoms and often reached to the leaf petiole even when symptoms were visible in <10% of the total leaf area. Comparisons of anthurium cultivars in which a nondestructive method was used to quantify the severity of leaf infection by a bioluminescent pathogen have enabled us to evaluate susceptibility and resistance to bacterial blight accurately. Such evaluations will be of importance in breeding resistant cultivars for disease control.     

Multiple components of the resistance of potatoes to potato leafroll virus

In glasshouse experiments the ranking of potato genotypes for resistance to infection with potato leafroll virus (PLRV) using three concentrations of aphid-borne inoculum was the same as their field resistance ratings. In field-grown plants this resistance to infection increased in all genotypes as the plants aged but its rate of increase differed between genotypes. In tests on field-grown plants infected by aphid- or graft-inoculation, the proportion of virus-free progeny tubers increased the later the date of inoculation but was greater in resistant than in susceptible genotypes. This trend was most pronounced in the resistant clone G7445(1), in which the virus failed to move from the foliage to the tubers of some plants infected in glasshouse tests. The spread of PLRV will thus be minimised in crops of resistant compared with susceptible genotypes for three reasons: plants have greater resistance to infection, systemic spread of virus from their foliage to tubers is less likely and, as shown previously, the low concentration of virus particles in leaf tissue makes infected plants less potent sources of inoculum for aphids.     

Stem canker (Rhizoctonia solani) of maincrop potatoes.

In two years, potato plants were sampled at 1- or 2- weekly intervals from plots planted with seed tubers bearing sclerotia of Rhizoctonia solani (black scurf) and with seed without sclerotia either infested or not with cultures of R. solani at planting. Sprouted King Edward seed was used in 1981 and sprouted and non-sprouted King Edward and Pentland Crown seed in 1982. In both years 60–80% of shoots from seed with sclerotia and 90% of shoots from seed inoculated at planting were affected with stem canker. Most disease developed before shoots emerged although it gradually increased later when new shoots arising both from seed tubers or as branches on shoots with damaged apices (pruned shoots) became infected before they emerged. Sprouting seed tubers bearing sclerotia decreased the disease on both cultivars but with soil-applied inoculum the disease was more severe on plants from sprouted than non-sprouted seed. Some stolons were infected by R. solani soon after they developed and incidence of infection later increased. Thirty to 50% of stolons were infected on plants from infected seed tubers and 60% on plants with soil-applied inoculum. With both cultivars and sources of inoculum about 70% of the infected stolons had their apices killed (pruned).     

Wyerone Acid Phytoalexin Synthesis and Peroxidase Activity as Markers for Resistance of Broad Beans to Chocolate Spot Disease

Infection of broad bean (Vicia faba L.) leaves with Botrytis fabae (Sard.) resulted in wyerone acid phytoalexin accumulation and increase in peroxidase activities in resistant and susceptible broad bean cultivars. Rapid wyerone acid accumulation was observed in leaves of resistant cultivars that reached levels greater than twofold of the susceptible cultivars. Following infection of broad bean cultivars, either resistant or susceptible to B. fabae, wyerone acid synthesis in resistant cultivars peaked at 5 days then declined, whereas in susceptible cultivars synthesis gradually decreased. While the ethanol‐extract of wyerone acid significantly reduced B. fabae spore germination with increased concentrations, the phytoalexin had no effect on in vitro B. fabae germ‐tube growth. Peroxidase activity in infected leaves of resistant was 10 times higher than the susceptible cultivars and eight times higher in uninfected leaves of the resistant than the susceptible cultivars. Peroxidase activity in infected leaves was greater than twofold in resistant cultivars and less than twofold in susceptible cultivars, when compared with uninfected leaves. The ratio of peroxidase activity in infected to uninfected leaves increased over time in susceptible cultivars but remained unchanged in resistant cultivars. Peroxidase activity in uninfected and infected leaves and wyerone acid biosynthesis in infected broad bean plants were successfully used to ascertain the resistance and susceptibility of four broad bean cultivars to B. fabae. Using wyerone acid synthesis and peroxidase activity as preliminary markers for resistance of broad bean to chocolate spot disease, caused by B. fabae, is suggested in this study.     

An analysis of responses of resistant and of susceptible tomato plants to Verticillium infection

Comparative studies were made of the responses of resistant and of susceptible Gem tomato plants to infection by Verticillium albo-atrum. When inoculated through roots, there were striking differences in their responses. In susceptible plants, the foliar symptoms and amount of mycelium in the stem increased rapidly for some time. Then the mycelium started to disappear from the stem; this was accompanied by a check to the normal progress of symptoms, and by the formation of tyloses. In resistant plants, a limited invasion of the root and lower stem was accompanied by rapid and extensive tylosis. The mycelium soon disappeared from the stem and the plant then recovered from the initially mild symptoms. There was an inverse relationship between the amount of mycelium and the extent of tylosis in infected plants. The growth of susceptible plants was markedly reduced by infection. Total leaf area was much less because the newly produced leaves did not expand normally. The root system in infected plants was smaller because there were many fewer tertiary roots. In resistant plants infection stimulated growth. Tomato cuttings inoculated with conidia reacted similarly to root-inoculated plants. Hyphae grew well in the vascular system of susceptible cuttings whereas in resistant cuttings the pathogen started to grow but soon disappeared. Detached leaves of susceptible plants, inoculated through cut ends, wilted more than did leaves from resistant plants. It is suggested that resistance is mainly of the active type that develops after infection.     

THE PATTERN OF FIELD INFECTION OF POTATO BY THE BEET RINGSPOT STRAIN OF TOMATO BLACK-RING, A SOIL-BORNE VIRUS

The potential importance of the beet ringspot strain of tomato black-ring, a soil-borne virus, was assessed by growing stocks of Kerr's Pink potato for 1 year on infested land and subsequently on uninfested land. The incidence of infection in two stocks was 39 and 8% in the first year on uninfested land, and 29 and 5% after 2 years.
The virus was usually restricted to the roots of plants in the first year of infection, but a few plants showed black rings and spots in their leaves. In the second year, 20–55% of the plants grown from tubers set by symptomless, but infected, mother plants were infected: many of these showed leaf necrosis, others had stunted shoots, and cupped and distorted leaves; some were symptomless although systemically infected. In the third and fourth years, most of the progeny from plants which had symptoms or which were symptomless but systemically infected, contained the virus: nearly all such infected plants were stunted and distorted or were symptomless. Infection decreased the weight of tubers produced by plants with severe necrotic spotting but not the yield of plants with less necrosis. The number and weight of tubers per plant were decreased by 15 and 20% respectively, in symptomless systemically infected plants, and by 20 and 30% in stunted plants.     

Stem canker (Rhizoctonia solani) on five early and seven maincrop potato cultivars I. Infection of shoots, stolons and tubers

In 1983 and 1984, potato seed tubers of five early and seven maincrop cultivars were inoculated with cultures of Rhizoctonia soluni during planting to simulate severe seed infection. Shoot and stolon infection was assessed in June-August and black scurf on tubers recorded after harvest in October. Almost all shoots of all cultivars had stem canker in both years and disease on shoots, stolons and tubers was more severe in 1984 than in 1983. In 1983 similar amounts of disease developed on all early cultivars and between 11% (Ulster Sceptre) and 32% (Maris Peer) shoots were pruned off. Maris Peer had a stem canker score lower than other cultivars in 1984 but more than half the shoots were pruned off. Shoot pruning on Estima, Ulster Prince and Ulster Sceptre was more common on plants from sprouted than non-sprouted seed. Between 30 and 50% of stolons were pruned off. After harvest in 1985, black scurf was least prevalent on Arran Comet and Maris Peer tubers and in 1984 on Arran Comet and Estima tubers from non-sprouted seed. Of the maincrop cultivars, King Edward plants from sprouted seed had many shoots pruned off in both years. Shoot pruning was also prevalent on Maris Piper and Pentland Squire plants from non-sprouted seed. Record had fewest pruned shoots and stolons and the lowest stem canker score. The disease was more severe on Pentland Crown and Maris Piper plants from non-sprouted than sprouted seed. Black scurf was most common on Cara and King Edward tubers in 1983 and on King Edward and Record tubers in 1984. In both years few shoots but many tubers were infected on plants from non-inoculated seed and the significance of this is discussed.     

Factors affecting the detection of potato leafroll virus in potato foliage by enzyme-linked immunosorbent assay

Using antiserum globulins that reacted only weakly with plant materials, potato leafroll virus (PLRV) at 10 ng/ml was detected consistently by enzyme-linked immunosorbent assay (ELISA). The reaction with PLRV particles was slightly impaired in potato leaf extracts that were diluted less than 10^-1 but not at greater dilutions. Antiserum globulins that reacted more strongly with plant materials could be used satisfactorily for coating microtitre plates but were unsuitable for conjugating with enzyme. The detection end-point of PLRV, in leaf sap of potato cv. Cara plants grown from infected tubers in the glasshouse, was about 10^-2 and the virus was reliably detected in extracts of composite samples of one infected and 15 virus-free leaves. PLRV concentration was much less in extracts of roots or stolons than in leaf extracts. The virus was detected in infected leaves of all 27 cultivars tested. PLRV was readily detectable 2 wk before symptoms of secondary infection developed in field-grown plants of cv. Cara and Maris Piper and remained so for at least 5 wk. Its concentration was slightly greater in old than in young leaves and was similar to that in glasshouse-grown plants. In field-grown plants of cv. Maris Piper with primary infection, PLRV was detected in tip leaves 21–42 days after lower leaves were inoculated by aphids; in some shoots it later reached a concentration, in tip leaves, similar to that in leaves with secondary infection. Symptoms of primary infection developed in the young leaves of some infected shoots but were inconspicuous and were not observed until at least a week after PLRV was detected by ELISA.     

Carbohydrates and resistance to <Emphasis Type="Italic">Phytophthora infestans</Emphasis> in potato plants

Plants generally deal with biotic or abiotic stresses by altering components as for example cell wall constituents and metabolites. Infection by Phytophthora infestans, the causal agent of late blight, constitutes a stress condition for the plants and they react to it with changes arising in their metabolism depending on the resistance level of the plants. The present work compares two potato hybrids differing in their level of horizontal resistance to late blight. Carbohydrate content in stems and leaves of infected and uninfected plants was determined by HPLC. Some carbohydrates accumulated in the stems of the resistant hybrid infected by P. infestans, whereas they remained unchanged in the susceptible hybrid. On the other hand, in the leaves, these carbohydrates accumulated only in the infected susceptible hybrid.     

Subterranean clover mottle sobemovirus: its host range, resistance in subterranean clover and transmission through seed and by grazing animals

Subterranean clover mottle sobemovirus (SCMV) was transmitted by manual inoculation of sap to 27 cultivars belonging to three sub-species of subterranean clover. The virus readily infected systemically all inoculated plants of five susceptible cultivars of ssp. subterraneum. Ten others showed partial resistance as not all infected plants developed systemic infection; cold winter conditions further delayed or prevented systemic movement in four of them. Two cultivars of spp. brachycalycinum and four of spp. yanninicum failed to develop systemic infection following inoculation and were considered highly resistant. Resistance to SCMV in three of the spp. yanninicum was further confirmed by the failure to establish detectable primary infections in most of the inoculated leaves. Moreover, when the four ssp. yanninicum cultivars were graft-inoculated with SCMV, systemic infection eventually developed in them but the virus concentration was low. SCMV was also transmitted by manual inoculation of sap to a further 23 species of Trifolium, Medicago or Pisum. Three species were non-hosts, five were infected only in inoculated leaves and 18 others developed systemic infection in some or all plants. SCMV reached very high concentrations and was stable in subterranean clover sap. It was transmitted experimentally between subterranean clover plants by brushing infected leaves against healthy ones and in swards was readily transmitted by the trampling and grazing of sheep, but only poorly by mowing. Seed transmission of SCMV to seedlings of five cultivars of subterranean clover was low (0–0.12%). SCMV was not transmitted by Myzus persicae.     

The Probability of Detecting Clavibacter michiganensis subsp. sepedonicus by Indexing Seed Potato Lots with Serological Tests

Detection of the bacterial ring pot pathogen ( Clavibacter michiganensis subsp. spedonicus ) in seed potato lots by laboratory indexing complements visual inspection. The probability of detecting symptomless infections is a function of sample size and incidence of infection. We determined the incidence of asymptomatic stem and tuber infections in four potato cultivars at three levels of inoculum. At the high inoculum level, 51–93% of stems were infected at 80 days after planting, and 10–59% of the tubers were infected at harvest. The effect of the different percentages of infected stems and tubers on the probability of detection for simple random sampling was calculated for a constant sample size. The actual detection levels for two cultivars planted in field plots with predetermined incidence levels of ring rot infected plants were reasonably close to predicted probabilities.     

Activity of Enzymes Related to H2O2 Generation and Metabolism in Leaf Apoplastic Fraction of Tomato Leaves Infected with Botrytis cinerea

Hydrogen peroxide generation rates of uninfected and infected leaves of two tomato (Lycopersicon esculentum) cultivars showing differential susceptibility to Botrytis cinerea were determined. The superoxide anion, hydroxyl radical, ascorbate contents and changes in NADH peroxidase, superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) activities in the apoplast fraction were analysed. Infected leaves had an increased hydrogen peroxide level. It was greater and generally occurred earlier in plants of the less susceptible cv. Perkoz than in those of the more susceptible cv. Corindo. Induction of nitrotetrazolium blue reducing activity and SOD levels in apoplast were higher in cv. Perkoz 24 h after inoculation. In the controls, NADH peroxidase activity in apoplast was higher in the more susceptible cv. Corindo, but after infection it increased faster and to a higher level in the less susceptible cv. Perkoz. NADH oxidation was inhibited by only 15% by a specific inhibitor DPI (diphenylene‐iodonium) but was completely inhibited by KCN and NaN₃. Similar increases in APX activity after 48 h and a small increase in catalase activities were observed in both cultivars soon after infection. These results indicate that resistance of tomato plants to infection by the necrotrophic fungus B. cinerea may result from early stimulation of hydrogen peroxide and superoxide radical generations by NADH peroxidase and SOD in apoplastic space, and they confirm the important role of their enhanced production in apoplastic spaces of plants.     

A method for assessing the field susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica)

In experiments to develop a method for assessing the field susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica) seed tubers were stab-inoculated near the stolon (attachment end), with a suspension of the bacterium, or with water, before planting. Disease symptoms were recorded in three years (1980–1982) and plant growth and yield in 1982. Estima and Maris Bard were the most susceptible cultivars with many plants failing to emerge and most of those that did showing disease symptoms. Pentland Crown was the most resistant: few plants failed to emerge and few showed blackleg. Nevertheless compared with water-inoculated plants bacterial inoculation of the seed tubers of this cultivar caused loss of yield and differences in tuber size distribution. Cara, Wilja and King Edward showed intermediate reactions.     

The effect of sowing date on adult plant resistance to Erysiphe graminis f.sp. avenae in oats

A technique developed for detecting small differences in mildew infection under glasshouse/laboratory conditions is described. A satisfactory correlation was found between results obtained by this method and infections in the field. Detailed examination of the levels of infection in cultivars Maldwyn and Milford showed that the upper and more recently developed leaves of Maldwyn had a significantly smaller area infected than those of Milford, although seedling leaves of both genotypes were very susceptible. This was most marked when plants were in the 6- to 7-leaf stage. Comparison of host plants sown on successive dates during the spring and early summer showed that with the acceleration of the rate of growth occurring in later-sown plants, resistance was expressed progressively earlier in the life of the plant. Newly expanded leaves which show very low level of infection at the 6- to 7-leaf stage become susceptible as they age, resistance being then expressed in the younger leaves.     

Influence of Meloidogyne incognita on the Content of Amino Acids and Nicotine in Tobacco Grown Under Gnotobiotic Conditions

Seedlings of Meloidogyne incognita-resistant (N.C. 95) and -susceptible (McNair 30) tobacco cultivars were grown aseptically for 55 days inside isolator chambers in autoclaved soil infested with 0 or 3,000 axenized eggs of M. im ognita per 500 cc of soil. Healthy and infected plants were compared. Dry root weights of infected plants of resistant and susceptible cultivars were 16% and 84%, respectively, less than the controls. Sixteen amino acids, including those precursors for nicotine, and nicotine, increased significantly in infected roots of both cultivars. Increases in amino acids in infected roots ranged from 28% for valine to 103% for tyrosine in the resistant N.C. 95, and from 30% for leucine to 148% for tyrosine in lhe susceptible McNair 30. Nicotine content (dry weight basis) increased 42% and 62% in infected roots of resistant and susceptible cultivars, respectively. Nematode infection increased nicotine by 112% in leaves of N.C. 95, and decreased it by 56% in leaves of McNair 30. Root damage by M. incognita probably decreased nicotine movement into leaves of McNair 30. In N.C. 95, nicotine movement into leaves apparently was not adversel b affected due to lack of significant root damage.     

Evaluating between-plant interference in field trials for assessing potato genotypes for resistance to late blight

Field trials were done with four cultivars over 3 years to assess the extent to which the amount of late blight on the foliage of a potato plant could be influenced by that on a neighbouring plant of the same or a different cultivar. Drills containing the test plants were interspersed with those of spreader plants (cv. King Edward) which were artificially inoculated with Phytophthora infestans. The intensity of blight on the test plants was recorded on several occasions.
Resistant cultivars tended to be scored as less resistant in mixtures with other cultivars than in pure stands, and susceptible cultivars tended likewise to be scored as more resistant in mixed stands. However, standard analysis of variance indicated no systematic evidence of a significant effect due to neighbour cultivars, nor of interaction between cultivars and neighbour cultivars. In contrast, Kempton's (1982) neighbour model indicated a significant and positive interference coefficient (β) in each trial, which generally decreased over time. Predicted pure stand scores for each cultivar indicated that the adjustment was greatest for the most resistant and most susceptible cultivars. There was no advantage in using two-plant rather than one-plant plots in withstanding neighbour effects.     

Tuber Blight Development in Potato Cultivars in Response to Different Genotypes of Phytophthora infestans

Migrations or introduction of new genotypes of Phytophthora infestans to a specific region imposes a different perspective for potato production. During 2009–2010, a late blight epidemic affected the Northeastern United States, which quickly spread through several states. The epidemic was characterized by the appearance of a new genotype of P. infestans designated US‐22, which was isolated from tomato and potato. Potato tubers are an essential component of late blight epidemics where the pathogen cannot overwinter on Solanaceous plants. Six potato cultivars were inoculated with 12 isolates of P. infestans (five different genotypes), including isolates of the genotype US‐22. Tuber blight development was characterized in terms of tissue darkening expressed as area under the disease progress curve values and lenticel infection. The responses indicated that US‐8 was more aggressive than US‐22, but US‐22 isolates obtained from potato were more aggressive on potato than those acquired from tomato. Tuber periderm responses to infection were limited, yet US‐8 isolates infected the periderm more often than US‐22 isolates. There were significant differences among the cultivars tested but cv. Jacqueline Lee was the most resistant overall. Although isolates of P. infestans genotype US‐22 were less aggressive in comparison with US‐8 isolates, US‐22 isolates still infected potato tubers and were as aggressive us US‐8 isolates on some cultivars. Management of late blight caused by isolates of US‐22 through host resistance may be feasible but imposes a different set of criteria for consideration from those that US‐8 imposed.     

Are Pathogenesis-Related Proteins Induced by Meloidogne javanica or Heterodera avenae lnvasion?

Changes in root- and leaf-soluble proteins were investigated in tomato after invasion by the root-knot nematode Meloidogyne javanica, or in barley and wheat after invasion by the cereal cyst nematode Heterodera avenae. Infection of susceptible tomato plants by M. javanica did not cause any change in the soluble-protein composition of leaves or roots compared with uninoculated plants at an early infection stage. No pathogenesis-related proteins (chitinase, glucanase, or P-14) were induced in the leaf apoplast. Changes in leaf proteins were not observed after invasion of wheat cultivars by H. avenae, whereas, in barley, a few changes in intercellular leaf proteins were recorded in resistant cultivars. These changes, however, were not the same among different H. avenae-resistant cultivars. Protein changes were found at an early stage of infection in barley and wheat roots infected with H. avenae, but no difference was found between resistant and susceptible cultivars.