Interaction between barley yellow dwarf virus and rust in wheat, barley and oats, and the effects on grain yield and quality

In three separate experiments, the upper leaf surface of the fifth formed leaf of wheat cv. Highbury, the fourth and fifth leaves of barley cv. Julia and the third and fourth leaves of oat cv. Mostyn were inoculated in a spore settling tower with wheat brown rust (Puccinia recondita f. sp. tritici), barley brown rust (P. hordei) or oat crown rust (P. coronata f. sp. avenae), respectively. Fewer pustules developed on distal portions of leaves of plants infected with barley yellow dwarf virus (BYDV) than on similar portions of leaves from virus-free plants. There were no significant differences in the number of pustules on proximal leaf portions. In barley and oats, the number of pustules on distal leaf portions was negatively correlated with the amount of yellowing of the leaf areas scored. In wheat, symptoms of BYDV were mild and leaves were little affected by yellowing. The latent period of rust on wheat and oats was not affected by BYDV. In barley, BYDV reduced the latent period of rust on leaf 5, but not on leaf 4, and reduced it on proximal, but not distal, leaf portions. In other experiments, BYDV reduced the yield of wheat and oats by 44% and 66%, respectively, while BYDV-infected barley was almost sterile. The appropriate rust reduced the yield of wheat, barley and oats by 33%, 13% and 86%, respectively. When infected with both BYDV and rust, yield of wheat and oats was reduced by 63% and 91%, respectively. Neither BYDV nor rust affected the percentage crude protein content of wheat grain, nor did rust affect that of barley. In oats, BYDV and rust each significantly increased crude protein of grain, but rust infection of BYDV-infected plants tended to reduce it.     

Effect of carbendazim and mancozeb combinationon Alternaria leaf blight and seed yield in sunflower (Helianthus annus L.)

Abstract

Leaf blight caused by Alternaria helianthi (Hansf.) Tubaki & Nishihara, is the major disease of sunflower affecting the successful cultivation across India. Five individual fungicides and two combination fungicides were evaluated against this pathogen in laboratory and in field experiments. Among them, the combination of carbendazim + mancozeb completely (100%) inhibited the mycelial growth of A. helianthi, irrespective of the concentrations tested followed by carbendazim alone and metalaxyl + mancozeb under in vitro condition. In field conditions, the combination of carbendazim + mancozeb was found to be highly effective in reducing the leaf blight disease of sunflower in all the three experiments as compared to other fungicides and unsprayed control. The reduction of Alternaria leaf blight was also directly associated with an increase in seed yield. The economics of the fungicides spray has been worked out and the benefit cost ratio for the combination of carbendazim + mancozeb at 2.0 g/l was 7.1 as compared to unsprayed control. The overall analysis of the results revealed that the combination of carbendazim + mancozeb at 2.0 g/l can be used for the management of foliar diseases such as Alternaria leaf spot/blight in agricultural crops.     

Identification and mapping of adult-onset sensitivity to victorin in barley

Victoria blight of oats caused by the fungus Cochliobolus victoriae is of distinct interest due to the link between Victoria blight susceptibility and crown rust resistance. C. victoriae-susceptible oats were introduced into the USA as a source of the Pc2 gene for resistance to the crown rust fungus Puccinia coronata. A dominant gene (Vb) in these oats was found to condition susceptibility to Victoria blight disease and sensitivity to the C. victoriae toxin called victorin. Numerous genetic approaches to separate Vb from Pc2 have failed, suggesting that Pc2 and Vb share identity. Because Victoria blight has only been described in allohexaploid oat, which has a poorly characterized genome of 11,300 Mb, molecular genetic investigations of Vb in oat are not practical. Previously we identified a presumed Vb ortholog in Arabidopsis, called LOV. LOV confers victorin sensitivity and susceptibility to C. victoriae, and encodes a coil-coil-nucleotide-binding site-leucine-rich repeat (CC-NBS-LRR) protein. Analysis of cereal DNA databases reveals a large array of CC-NBS-LRR genes, but no obvious LOV ortholog. Identifying a cereal ortholog of LOV will require identification and subsequent mapping of victorin sensitivity in a genetically tractable cereal plant. In this work, we surveyed barley for victorin sensitivity and identified adult-onset sensitivity to victorin in eight barley accessions. Evaluation of a doubled haploid (DH) population derived from the cross of sensitive × insensitive parents revealed a single quantitative trait locus (QTL) for victorin sensitivity in a resistance-gene-rich region on the short arm of chromosome 1H. Furthermore, enhanced victorin sensitivity observed in some DH lines suggests a background-dependent enhancement of victorin sensitivity.     

Studies of the incidence, severity and distribution of false rust (Synchytrium psophocarpi), leaf spot (Pseudocercospora psophocarpi) and powdery mildew (Oidium sp.) on winged bean (Psophocarpus tetragonolobus) in Papua New Guinea

The incidence, severity and distribution of false rust, (Synchytrium psophocarpi), leaf spot (Pseudocercospora psophocarpi) and powdery mildew (Oidium sp.) on winged beans (Psophocarpus tetragonolobus) were evaluated on pure lines and mixed stands grown in Papua New Guinea between 1976 and 1978. Almost all of the 148 pure lines were susceptible to false rust and leaf spot and the incidence of these diseases increased with plant age. Standard area diagrams were superior to disease keys in evaluating disease severity. Levels of false rust on 10 promising agronomic lines differed according to the locality where the lines were grown. Two Thai lines showed resistance to false rust but P. scandens was immune to false rust and leaf spot. Surveys of 51 sites containing mixed stands of winged beans in the Eastern Highlands, Western Highlands, East New Britain, East Sepik, Madang and Morobe Provinces of Papua New Guinea indicated that leaf spot and false rust were present in both the wet and dry seasons and the apparent absence of disease was probably due to time and size of samples. The diseases appeared to be present in all the traditional winged bean growing areas of Papua New Guinea. It was difficult to obtain evidence of differences in palatability of tissues infected with false rust. The value of surveys and disease assessment in relation to evaluation of crop losses of winged bean caused by these pathogens and selection for resistance is discussed.     

Temperature and Leaf Wetness Effects on Infection of Sugarcane by Puccinia melanocephala

Brown rust epidemics in sugarcane, caused by Puccinia melanocephala, vary in severity between seasons. To improve the understanding of disease epidemiology, the effects of leaf wetness, temperature and their interaction on infection of sugarcane by the pathogen were studied under controlled conditions. Disease severity was low at 15 and 31°C regardless of leaf wetness duration. No infection occurred with a 4‐h leaf wetness period. Increasing leaf wetness duration from 7 to 13 h lowered the temperature required for disease onset from 21 to 17°C. More infection occurred with 13 compared to 10 h of leaf wetness at 17°C, and severity decreased for all leaf wetness periods at 29 compared to 27°C. Postinfection suboptimal low and high temperatures increased the time required for lesion development and high temperatures decreased maximum disease severity. The observed effects of leaf wetness and temperature on infection by P. melanocephala could help explain the initiation, rate of increase and decline of brown rust epidemics in the field.     

Phenotyping of Partial Physiological Resistance to Rice Sheath Blight

Sheath blight, caused by Rhizoctonia solani, is one of the most important rice diseases worldwide especially under irrigated agro‐ecosystems. To date, no rice accession with complete resistance to sheath blight has been reported. However, a number of genotypes with varying levels of resistance have been reported. Twelve genotypes (including mega varieties) viz. Tetep, Jasmine 85, Te‐Qing, Duduruchi, Betichikon, Khatochalani, D‐6766, D‐256, Swarna, Sarju‐52, MTU‐1010 and Samba Mashuri were evaluated for quantitative measurement of partial physiological resistance to sheath blight under controlled conditions using detached tiller method. Three independent experiments, each involving three replications, were conducted. Seven days after inoculation, the following disease variables were measured: number of lesions, lesion length, vertical sheath colonization (VSC) on the tiller, disease severity, relative vertical sheath colonization (RVSC) and survival of the leaf blade. Variation between rice genotypes was observed for all the disease variables. Disease severity and VSC were the two most correlated variables, whereas the number of lesions and mean lesion length were the least correlated variables. The ranking of varieties often differed depending on the disease variable considered. Amongst the genotypes tested, D‐256, Tetep and Jasmin‐85 had the lowest number of lesions and disease severity. Similarly, Tetep and D‐256 showed the lowest levels of RVSC, whilst Jasmine‐85 was found to be intermediate. D‐6766, Samba Mashuri and Betichikon showed the highest levels of disease variables. The fraction of dead leaves ranged from 0.00 to 0.38. No dead leaves were observed in Te‐Qing, Swarna and MTU‐1010. The highest fraction of dead leaves was observed for Betichikon (0.38) followed by Duduruchi and D‐6766 (0.33). Our results suggest that this method in combination with other phenotyping methods could be used to quantify partial resistance to rice sheath blight.     

Incidence of crown rust and virulence of puccinia coronata cda. f.sp. avenae eriks. and the effectiveness of pc genes for resistance in europe,middle east and north africa: Inzidenz von kronenrost und virulenz von puccinia coronata cda. f.sp. avenae eriks. und die wirksamkeit von pc-genen für resistenz in europa,nahost und nordafrika

Crown rust on oat incited by Puccinia coronata Cda f.sp. avenae is a wide-spread disease in Europe, Middle East (Israel) and North Africa (Morocco). High natural levels of the disease were recorded in Austria, Bulgaria, Czech Republic, Estonia, Israel, Italy, Poland, Russia and the former Yugoslavia. The severity of the disease in the European and Mediterranean Oat Disease Nursery (EMODN) between 1995 and 1999 showed that it occurred at damaging levels in a number of countries. Considerable differences in a disease resistance index (DRI) to crown rust among 67 oat lines in the period 1995?–?2000 were found. The values of the DRI ranged from18 (KR 8122) to over 290 (Pc 59, Pc 68). Detailed studies of the virulence combinations of P.coronata f.sp. avenae in Europe, Middle East and North Africa were carried out between 1995?–?2001. There were considerable differences in the average number of virulence among countries. Virulence to Pc 39 and 68 was found for the first time and is a significant finding. Nevertheless the major genes Pc 68, Pc 39, Pc 50-2, Pc Pc 59, Pc 60, Pc 61 proved to be highly effective.     

Puccinia coronata f. sp. avenae: a threat to global oat production

Puccinia coronata f. sp. avenae (Pca) causes crown rust disease in cultivated and wild oat (Avena spp.). The significant yield losses inflicted by this pathogen make crown rust the most devastating disease in the oat industry. Pca is a basidiomycete fungus with an obligate biotrophic lifestyle, and is classified as a typical macrocyclic and heteroecious fungus. The asexual phase in the life cycle of Pca occurs in oat, whereas the sexual phase takes place primarily in Rhamnus species as the alternative host. Epidemics of crown rust happens in areas with warm temperatures (20–25 °C) and high humidity. Infection by the pathogen leads to plant lodging and shrivelled grain of poor quality. Disease symptoms : Infection of susceptible oat varieties gives rise to orange–yellow round to oblong uredinia (pustules) containing newly formed urediniospores. Pustules vary in size and can be larger than 5 mm in length. Infection occurs primarily on the surfaces of leaves, although occasional symptoms develop in the oat leaf sheaths and/or floral structures, such as awns. Symptoms in resistant oat varieties vary from flecks to small pustules, typically accompanied by chlorotic halos and/or necrosis. The pycnial and aecial stages are mostly present in the leaves of Rhamnus species, but occasionally symptoms can also be observed in petioles, young stems and floral structures. Aecial structures display a characteristic hypertrophy and can differ in size, occasionally reaching more than 5 mm in diameter. Taxonomy : Pca belongs to the kingdom Fungi, phylum Basidiomycota, class Pucciniomycetes, order Pucciniales and family Pucciniaceae. Host range : Puccinia coronata sensu lato can infect 290 species of grass hosts. Pca is prevalent in all oat‐growing regions and, compared with other cereal rusts, displays a broad telial host range. The most common grass hosts of Pca include cultivated hexaploid oat (Avena sativa) and wild relatives, such as bluejoint grass, perennial ryegrass and fescue. Alternative hosts include several species of Rhamnus, with R. cathartica (common buckthorn) as the most important alternative host in Europe and North America. Control : Most crown rust management strategies involve the use of rust‐resistant crop varieties and the application of fungicides. The attainment of the durability of resistance against Pca is difficult as it is a highly variable pathogen with a great propensity to overcome the genetic resistance of varieties. Thus, adult plant resistance is often exploited in oat breeding programmes to develop new crown rust‐resistant varieties. Useful website : https://www.ars.usda.gov/midwest-area/st-paul-mn/cereal-disease-lab/docs/cereal-rusts/race-surveys/ .     

Relationship between onion thrips (Thrips tabaci) and Stemphylium vesicarium in the development of Stemphylium leaf blight in onion

Stemphylium leaf blight caused by Stemphylium vesicarium and onion thrips (Thrips tabaci) are two common causes of leaf damage in onion production. Onion thrips is known to interact synergistically with pathogens to exacerbate plant disease. However, the potential relationship between onion thrips and Stemphylium leaf blight is unknown. In a series of controlled laboratory and field trials, the relationship between thrips feeding and movement on the development and severity of Stemphylium leaf blight were examined. In laboratory assays, onions (“Avalon” and “Ailsa Craig”) with varying levels of thrips feeding damage were inoculated with S. vesicarium. Pathogen colonisation and leaf dieback were measured after 2 weeks. In pathogen transfer assays, thrips were exposed to S. vesicarium conidia, transferred to onion and leaf disease development was monitored. In field trials, insecticide use was examined as a potential indirect means to reduce Stemphylium leaf blight disease and pathogen colonisation by reducing thrips damage. Results from laboratory trials revealed that a reduction in thrips feeding decreased S. vesicarium colonisation of onion leaves by 2.3–2.9 times, and decreased leaf dieback by 40–50%. Additionally, onion thrips were capable of transferring S. vesicarium conidia to onion plants (albeit at a low frequency of 2–14% of plants inoculated). In field trials, the symptoms and colonisation of Stemphylium leaf blight were reduced by 27 and 17%, respectively with the use of insecticide to control thrips. These results suggest that onion thrips may play a significant role in the development of Stemphylium leaf blight, and thrips control may reduce disease in commercial onion fields.     

Interaction of Apple Cultivars with Populations of Cedar-Apple Rust

Five physiological populations of Gymnosporangium juniperi-virginianae Schw. were identified on the basis of pycnial and aecial lesion formation when seven apple cultivars grown in a glasshouse were inoculated with basidiospores of rust populations collected from six locations in the U.S.A. The relationship between cultivars and rust populations was explored quantitatively using the parameters N, D, and DR, where N was the highest number of pycnial lesions on a leaf/plant; D was the largestdiameter measurement of pycnial lesions on a leaf/plant; and DR, the disease rating, was proportional to the diseased area of the leaf and calculated as ND². Statistically significant differences among cultivars over all rust populations for variables N, D, and DR were obtained, indicating the possible existence of race non-specific resistance in apple to cedar-apple rust. Moreover, significant differences among the rust populations over all cultivars for variable N and DR were obtained, confirming the existence of physiologically different rust populations of the pathogen. A change in the pathogenicities of these physiological populations must have occurred over time (10-year period). A significant interaction between cultivars and rust populations was observed for lesion size (D) only, indicating a probable existence of race specific resistance in apple as well.     

Number of genes controlling slow rusting resistance to leaf rust in five spring wheat cultivars

The number of genes controlling slow rusting resistance to leaf rust (Puccinia triticina) was estimated in five spring wheat (Triticum aestivum) cultivars using quantitative formulae. Parents and F₆ families were evaluated in replicated field trials under epidemics initiated by artificial inoculation. The F₆ families resulted from a diallel cross involving the fast-rusting cultivar Yecora 70 and five slow-rusting wheat cultivars: Sonoita 81, Tanager ‘S’, Galvez 87, Ures 81, and Moncho ‘S’. The area under the disease progress curve (AUDPC) was used to measure leaf rust severity over time. Results indicate that cultivar Sonoita 81 has three or four genes, Tanager ‘S’ has two or three genes, Galvez 87 has three genes, and both Ures 81 and Moncho ‘S’ have two genes for slow rusting resistance to leaf rust. Based on this result and previously reported moderate to high narrow-sense heritability estimates for slow rusting resistance in these materials, early-generation selection for slow leaf rusting would be effective.     

Undescribed wheat gene for partial leaf rust and stripe rust resistance from Thatcher derivatives RL6058 and 90RN249 carryingLr34

Inheritance of partial leaf rust and stripe rust resistance of a Thatcher wheat 90RN2491, earlier reported to carry two doses of the gene pairLr34-Yr18 and the reference line RL6058 (6*Thatcher/PI58548) for theLr34-Yr18 gene pair was studied against predominant and highly virulent Indian races. Thatcher derivatives 90RN2491 and RL6058 were intercrossed as well as crossed with the leaf rust and stripe rust susceptible Indian cultivar WL711. The F₁, F₂ and F₃ generations from these crosses were assessed for rust severity against leaf rust race 77-5 and stripe rust race 46S119. The F₂ and F₃ generations from the crosses of RL6058 and 90RN2491 with WL711, segregated 15 resistant : 1 susceptible (F₂) and 7 homozygous resistant : 8 segregating : 1 homozygous susceptible (F₃) ratios, respectively, both for leaf rust and stripe rust severity. Therefore, partial resistance against each of the leaf rust and stripe rust races in both RL6058 and 90RN2491 is ascribed to two independently inherited dominant genes. One of the two genes for leaf rust and stripe rust resistance in 90RN2491 and RL6058 isLr34 and the linked geneYr18, respectively. The second leaf rust resistance gene in both the Thatcher lines segregated independently of stripe rust resistance. Therefore, it is notLr34 and it remains unidentified.     

新疆油用型向日葵种质资源对2种新入侵病害的抗性鉴定与评价

【目的】向日葵白锈病、黑茎病是2种危害严重的新入侵病害,新疆地区新源县和特克斯县为这2种病害发生区。研究目前种植的油用型向日葵品种对向日葵白锈病和黑茎病的抗性差异,可为新疆伊犁地区向日葵品种选育及其病害综合防控提供理论依据。【方法】选择伊犁地区广泛种植的16种油用型向日葵种质资源为供试品种,试验地设在新源县和特克斯县田间,于2015年5—9月调查样地的病叶数,并计算病情指数,同时测定样地的平均产量,最后综合以上结果确定各品种的抗性水平。【结果】不同向日葵品种在同一供试条件下对白锈病和黑茎病存在明显的抗性差异,在新源县NX19012(西域朝阳)的病情指数分别为7.83和25.62,而品种KWS204的病情指数为21.04和39.50;部分品种在不同的供试地点表现出一致的抗性水平,品种新引S31感染白锈病后在新源县的病情指数为9.41,在特克斯县为9.38;另有部分品种在不同地点存在不同的抗性水平,如G101感染白锈病后在新源县和特克斯县的病情指数分别为9.16和11.78,而感染黑茎病后在新源县和特克斯县的病情指数分别为31.05和37.79。【结论】通过对白锈病和黑茎病病情指数、向日葵产量及当地气象数据的综合研究,发现NX19012(西域朝阳)最适宜在新疆伊犁地区种植,品种S606、TO12244、NX01025(西部骆驼)也可作为多样化种植的候选品种,而新葵杂系列品种中新葵杂5号可作为今后品种选育的试验材料。     

Effects of Epoxiconozale and Pyraclostrobin Fungicides in the Infection Process of Hemileia vastatrix on Coffee Leaves as Determined by Chlorophyll a Fluorescence Imaging

Coffee is the most traded commodity in the world, and Brazil is its largest producer. Coffee leaf rust, caused by the biotrophic fungus Hemileia vastatrix, is the most important coffee disease, reducing coffee yield by 35–50%. This study aimed to use the ratio of variable and maximum fluorescence of dark‐adapted tissue (F_v/F_m) as a parameter to differentiate presymptomatic tissue from healthy tissue during disease development in plants sprayed with pyraclostrobin and epoxiconazole after 4 days postinoculation. Visual severity was considered as an indicative of apparent disease and true severity as an indicative of both apparent and non‐apparent disease. There was a significant linear relationship between the areas of true severity and visual severity, and for each additional unit in the visual severity, there was an increase of 1.53 units on the true severity. For the epoxiconazole and pyraclostrobin treatments, coffee leaf rust symptoms decreased according to both visual and F_v/F_m images. Pustules on the leaves sprayed with epoxiconazole were smaller in size than those on the leaves of non‐sprayed plants but bigger than those sprayed with pyraclostrobin. The reduction in F_v/F_m values at the pustule epicentres present on the leaves of plants sprayed with epoxiconazole, and pyraclostrobin was greater than those of the non‐sprayed plants. This finding was expected and reflects the importance of these fungicides in prohibiting the progress of coffee leaf rust. The photosynthetic capacity of Coffea arabica was affected by H. vastatrix infection, and the F_v/F_m parameter was able to show this effect before the visual symptoms were noticed.     

Identification of a RAPD marker linked to the oat stem rust gene Pg3

Summary The feasibility of identifying molecular markers linked to disease resistance genes in oats was investigated utilizing random primers in conjunction with polymerase chain reaction technology. A pair of near-isogenic oat lines were screened for polymorphic DNA fragments linked to the stem rust resistance gene Pg3. Two primers were identified which amplified DNA fragments that were polymorphic between the lines analyzed. One primer (ACOpR-2) was shown to be completely linked to the Pg3 locus; the other primer was not linked to either the ACOpR-2 or the Pg3 loci. This type of analysis, combined with rapid leaf disc DNA extraction techniques, offers an effective means of identifying useful molecular markers and of applying them to plant breeding selection strategies.Plant Research Centre publication number: 1443     

Effectiveness of acibenzolar-S-methyl, fungicides and antibiotics for the control of brown eye spot, bacterial blight, brown leaf spot and coffee rust in coffee

This study was carried out to evaluate the potential of acibenzolar‐S‐methyl (ASM), combined or not combined with fungicides and antibiotics for the control of brown eye spot (Cercospora coffeicola) and bacterial blight (Pseudomonas syringae pv. garcae) in coffee seedlings, and ASM combined with conventional fungicide application schedules for the control of coffee rust (Hemileia vastatrix) and brown leaf spot (Phoma costarricencis) under field conditions in two coffee crops in the State of São Paulo, Brazil. ASM protected coffee seedlings against C. coffeicola when applied at the rates of 2.5 and 5 g of active ingredient per hectolitre of water (g a.i. hL^?1), providing 34–55% of disease control, and against bacterial blight, when applied at the rates of 2.5, 10 and 20 g a.i. hL^?1, with 38–57% of disease control. Tebuconazole (100 g a.i. hL^?1) and azoxystrobin (10 g a.i. hL^?1) showed the best results for brown eye spot control. Oxytetracycline + streptomycin, kasugamycin hydrochloride, oxytetracycline + metallic copper, copper oxychloride and mancozeb + copper oxychloride also controlled bacterial blight in levels similar to those shown by ASM. In the field experiments, all fungicide application schedules tested, cyproconazole (December, February, April), epoxiconazole (December, March), tetraconazole (December, February, April), cyproconazole (December, February) and azoxystrobin (January, March) were effective for coffee rust control and provided partial control of brown leaf spot. The results also showed that for all experiments, there was no synergistic effect of the combination of ASM with azoxystrobin, cyproconazole or cupric fungicides.     

Determination of Physiological Races and Evaluation of Sunflower for Resistance to Puccinia helianthi Schw

Sunflower rust, caused by Puccinia helianthi Schw., is a widespread disease of sunflower (Helianthus annuus L.) in China. To study physiological races, sunflower field surveys were undertaken in major sunflower growing areas of China in 2010. Forty‐four rust‐infected sunflower leaf samples were collected from 25 geographical locations. Freshly produced spores were used to study physiological race differentiation on a set of nine differentials. Race 300 was the most prevalent race observed over all locations with a 59% frequency followed by races 735, 310, 500, 724 and 737. To evaluate hybrids and varieties for resistance screening, spores of race 300 were used to inoculate 65 hybrids, and five open‐pollinated varieties selected from breeding programmes and from the seed market. None of the confection hybrids and open‐pollinated varieties was immune to race 300. Conversely, among oilseed hybrids, 3% of them showed immunity, 12% highly resistant, 59% resistant and 26% showed susceptible reactions. Open‐pollinated varieties were the most susceptible to race 300 followed by confection and oilseed sunflower hybrids. Results from this study are projected to assist breeders in selection of hybrids and varieties against prevalent race as our results showed a diversity of resistance levels to race 300.     

The effect of early blight disease caused by Alternaria solani on shoot growth of young tomato plants

The effect of increasing spore concentration of Alternaria solani (Early blight disease) on the shoot growth of young tomato plants was analysed. Changes in growth were related to the severity of infection which increased with increasing inoculum. Leaf production was not affected but dry weights and especially leaf expansion were decreased. The effective leaf areas of the five inoculated leaves (L1-L5 numbered from the plant base) were drastically decreased by expanding necrotic lesions and, to a lesser extent, by premature leaf fall. Healthy leaves expanding soon after inoculation (L6, L7) were markedly affected by the disease on the lower leaves and had decreased specific leaf areas (ratio of leaf area to leaf dry weight) but later formed (from L8) leaves were less affected and had greater specific leaf areas than equivalent leaves on uninoculated plants.     

Screening for fire blight resistance in apple (Malus pumila) using excised leaf assays from in vitro and in vivo grown material

The apple cultivars Greensleeves and Novole and somaclones derived from Greensleeves were tested for resistance to fire blight (Erwinia amylovora) using excised leaf bioassays. Cv. Greensleeves was consistently susceptible to fire blight in in vitro, and in glasshouse and excised leaf bioassays whilst cv. Novole was resistant. Greensleeves somaclones, identified as having intermediate levels of fire blight resistance in glasshouse and in vitro screening tests, also showed intermediate levels of resistance in excised leaf bioassays. The length of leaves and the inoculum concentration employed affected the severity of symptoms observed. Excised leaf bioassays were unsuitable as tests for fire blight resistance using field grown material.     

Efficacy of fungicide combinations,phosphoric acid and plant extract from stinging nettle on potato late blight management and tuber yield

Late blight caused by Phytophthora infestans is a major constraint to potato production. Inadequate control of the disease has often resulted in potato yield losses. We assessed the efficacy of fungicides, phosphoric acid and stinging nettle extract combinations for late blight control at two locations in Kenya. Disease severity, relative area under disease progress curves (RAUDPC), pathogen lesions and tuber yield were quantified during the 2008 and 2009 cropping cycles. The application of metalaxyl alternated with phosphate resulted in the greatest suppressive effects on late blight. The average late blight severity ranged from 3.5 to 34% in 2008 and 4.7 to 50% in 2009 at Tigoni location. RAUDPC for the same location ranged from 5 to 40% and 5 to 50% in 2008 and 2009, respectively. Similar levels of late blight severity were recorded at Marimba location in both years. Lesion growth and pathogen lesion numbers on potato plants differed significantly (p < 0.05) among treatments. Fungicides, phosphoric acid and stinging nettle extract varied in late blight control. Potato tuber yield varied among treatments. Phosphoric acid treatment had significantly (p < 0.05) greater tuber yield compared to metalaxyl at both locations. Field plots treated with plant extracts from stinging nettle resulted in the lowest tuber yield compared to other treatments with the exception of the untreated control. Fungicides, phosphoric acid, stinging nettle extract and their combinations can be readily effective in the suppression of late blight severity and pathogen lesions with moderate increases in tuber yield.