I. Pathogenic variation in fungi and bacteria and mycorrhizal compatibility: Pathogenic variation in Verticillium albo-atrum isolates from hop (Humulus lupulus)

Non-lethal (M strain) ‘fluctuating’ wilt was first identified in hops in 1924. Lethal (V strain) ‘progressive’ wilt appeared in the 1930s and spread rapidly through Kent, destroying all commercial varieties. By 1965 the disease was largely controlled by use of new resistant cultivars, accompanied by restricted nitrogen fertilisers, non-cultivation (with weed control by overall herbicides), and high standards of field hygiene. Between 1968 and 1971 severe wilt developed in previously highly resistant clones, used as ‘controls’ in routine field resistance tests. Verticillium isolates from the two farms supplying the natural inoculum for these tests during 1968–71 were more virulent than previously encountered V strains, some causing severe wilt in all resistant cultivars. Subsequent surveys supported by pathogenicity tests against resistant cultivars indicated the existence of two ‘super virulent’ strains on eight farms in Kent. The appearance of mild (M) strains in the 1920s, virulent (VI) strains in the 1930s, and ‘super-virulent’ (V2 and V3) strains in the 1960s follow 400 years of apparent freedom from wilt. This suggests that the initial breakage of resistance barriers was critical and subsequent variations in an apparently complex, polygenically controlled resistance system are rapidly being matched by the pathogen.     

Integrated management of bacterial wilt (Ralstonia solanacearum) of ginger (Zingiber officinale) in Southwestern Ethiopia

Abstract

Ginger bacterial wilt (GBW) is a destructive disease of ginger in Ethiopia. Field studies were conducted to determine effect of integrated management of GBW, through host resistance and cultural practices, on wilt epidemics at Teppi and Jimma, southwestern Ethiopia in 2017. Treatments were factorial arranged in a randomised complete block design with three replications. Analysis of variance indicated that effect of variety, cultural practices and variety x cultural practice interactions significantly reduced GBW epidemic parameters and enhanced yield at both locations. Boziab recorded lower wilt incidence, area under disease progress curve and wilt progress rates than Local variety. Integration of lemon grass with soil solarisation and fertiliser reduced wilt incidence in Local variety up to 38.3% (Teppi) and 42.05% (Jimma) compared to the control on final wilt assessment date. In Boziab variety, integrated use of lemon grass with soil solarisation and fertiliser reduced wilt incidence up to 42.5% at Teppi and 33.85% at Jimma compared to the control on final date of wilt assessment. The overall results revealed that integrating cultural practices with host resistance are found effective to slow down GBW epidemics and improve ginger productivity; and thus, recommended for the study areas along with other crop management practices.     

Verticillium wilt of the hop: field studies on wilt in a resistant cultivar in relation to nitrogen fertilizer applications

In two large-scale observation trials on commercial farms inorganic nitrogen fertilizer was applied at 70 lb. N/acre (N1), 1401b. N/acre (N2) and at a normal commercial rate of 210 lb. N/acre (N3). Mean wilt incidence was 60 and 25% less with Ni and N2 respectively than with N3 during 5 years at one site and 3 years at the other. During these periods wilt declined progressively and this was tentatively attributed to the reduced intensity of host colonization, and to progressive declines in soil infectivity resulting from effects on the quantity, quality and longevity of annually produced inoculum. Marked annual fluctuations of wilt incidence and yields occurred at one site and these were associated with weather in the spring and early summer: wilt incidence was inversely related to soil temperature, and yield was directly related to rainfall. Yields were not diminished by the low-N treatments, but clear correlations between yields and wilt incidence were probably obscured by the differential effects of weather conditions at the three levels of N application. The results emphasized the importance of reducing traditionally high N applications and, on farms where fluctuating wilt is severe, of applying the minimum levels of N commensurate with the maintenance of satisfactory yields.     

An update on control of brown spot of pear

Brown spot of pear is a fungal disease producing high economical losses in several pear-growing areas in Europe. Fungicide applications during the growing period either at fixed schedule or delivered according to the BSPcast forecasting system are not enough to control the disease under favorable conditions. New strategies have been introduced to control the inoculum production using sanitation methods. These methods are based on combinations of leaf litter removal during winter and biological control agent applications during late winter, spring and summer. These practices reduce both the inoculum pressure and disease levels. Therefore, the resulting optimized disease management consists of a combination of sanitation methods applied during the whole year with chemical fungicides scheduled according to the BSPcast forecasting model during the vegetative period. It is expected that the control of brown spot could be further refined upon availability of rapid methods for inoculum potential analysis. However, this analysis is difficult due to the variability in pathogenicity within the pathogen population.     

Population dynamics of Frankliniella spp. and tomato spotted wilt incidence as influenced by cultural management tactics in tomato

We investigated the effects of ultraviolet (UV)-reflective mulch and two rates of nitrogen fertilization on populations of Frankliniella spp. thrips and on the incidence of tomato spotted wilt in field-grown tomato in northern Florida. The higher of the two soil nitrogen fertilizer treatments significantly increased populations of Frankliniella occidentalis (Pergande), whereas mulch reflectance significantly decreased populations of F. occidentalis. Populations of Frankliniella tritici (Fitch) were decreased only by UV-reflective mulch. Decreased thrips populations in UV-reflective mulch plots were probably due to disruptions in host-finding behavior. Increased thrips populations in tomatoes treated with the higher nitrogen fertilization rate were probably due in part to increased nutrients available in flowers. Incidence of tomato spotted wilt was significantly decreased in tomatoes grown on UV-reflective mulch, whereas disease incidence was significantly greater in increased nitrogen-fertilized plots. This research reveals that cultural practices resulted in up to 45% reduction in the numbers of vector and nonvector species of flower thrips and up to 50% reduction in tomato spotted wilt.     

Spatial distribution and association of banana (Musa spp.) Fusarium wilt (Fusarium oxysporum f.sp. cubense) epidemics with biophysical factors in southwestern Ethiopia

Abstract

Field surveys were conducted in 2014 and 2015 to determine the distribution and association of wilt incidence with biophysical factors. Spatio-temporal analysis of wilt epidemics in the farms revealed a wide distribution of the disease. Cropping year, district, cultivar, crop density, planting date, weeding, sucker and bud management, source of moisture and planting materials, presence/absence of wilt in neighboring farms, disease management practices, farmers awareness towards wilt diseases, planting date and cropping system were significantly associated with disease incidence in a multiple-variable model. A wilt incidence of ≤30% had high probability of association to good weed, sucker and male bud management practices, high level of farmers awareness towards wilt, low population density, disease-free suckers, rainfed production, absence of wilt in neighboring farms and good disease management schemes. Findings of this study showed Fusarium wilt is a major constraint and need for on-farm trainings to concerned bodies about wilt management schemes.     

Survival ofPseudomonas solanacearum in soil

Summary Studies on the survival ofPseudomonas solanacearum, the causal agent of bacterial wilt of tomato, under laboratory conditions showed that soil amendments had little effect on the population of the pathogen. When the host plant was grown in amended soil there was a positive influence on growth of the pathogen. The population level of the pathogen incorporated into the soil was reduced to one-half within a period of 46 days. The pathogen survived in the rhizosphere of non-host plants belonging to the families Acanthaceae and Leguminosae even in the absence of the natural hosts, but its incidence in the rhizosphere of plants belonging to Graminae and Cyperaceae was comparatively low indicating possibilities of reducing the inoculum potential of the pathogen in tomato fields by allowing such plants to grow during fallow periods. Part of the thesis of the Senior Author presented to the Kerala Agricultural University, Trichur, Kerala State, India.     

Verticillium wilt of the hop: fluctuating outbreaks in wilt-resistant varieties

New varieties of hop resistant to progressive Verticillium wilt are now widely grown and generally give acceptable disease control in commercial plantations, but on a few farms their normal high level of resistance is not maintained. The incidence of wilt on one such farm is analysed in detail. At this site wilt was not directly associated with plant death, as it is in a sensitive variety. In a discussion on etiological aspects of wilt in resistant varieties it is concluded that while seasonal fluctuations in disease incidence are influenced by climatic conditions, high mean incidence is related to undefined, intrinsic soil factors. Attention is directed to the analogous forms of disease resulting from host/pathogen interactions in which resistance and virulence are either both high or both low.     

The reactions of susceptible and resistant tomato cultivars to strains of Verticillium albo-atrum

Three strains of Verticillium albo-atrum causing severe wilt of tomato (T), progressive (Hp ) and fluctuating (HF) wilt of hop, were inoculated through the roots of four tomato cultivars at different inoculum concentrations. Symptoms were assessed visually 42 days after inoculation, and quantitatively on the change in total leaf area compared with controls. Distribution of mycelium and tyloses was determined by sections at 2 cm intervals of root, stem and petiole. Cultivars Loran Blood and Moscow showed resistance to disease expression at all levels of inoculum concentration with the T strain. Bonny Best and Potentate were both susceptible to this strain, but whereas in Potentate, disease severity increased from mild to severe with increase in inoculum concentration, Bonny Best was severely diseased at the lowest level of inoculum. All cultivars showed some susceptibility to the HP and HF strains; the ‘resistance’ of Loran Blood and Moscow was no longer apparent and Bonny Best was most severely affected. The relative susceptibilities to the strains were HF Bonney Best > Loran Blood > Potentate > Moscow, HP Bonny Best > Loran Blood, Moscow > Potentate, T Bonny Best > Potentate > Loran Blood, Moscow. In general, vascular colonization was less in the cultivars Loran Blood and Moscow with all three fungal strains at io⁵propagules/ml level of inoculum, but this was not always correlated with an increase in disease severity. With the exception of the host-pathogen combinations Bonny Best/T, Bonny Best/HF, Potentate/T and Moscow/T, increasing the inoculum concentration to 10⁷propagules/ml increased disease severity but had little or no effect of increasing vascular colonization. In Bonny Best/T, Bonny Best/HF and Potentate/T vascular colonization was reduced with the higher level of inoculum. Moscow showed complete resistance to symptom expression and little vascular colonization with the T strain at 10⁵prop./ml. At 10⁷prop./ml resistance was maintained but there was very extensive growth of mycelium in the vessels. Tylosis resulted from an interaction of host, fungal strain and the level of inoculum and was not always correlated with the degree of vascular colonization. Contrary to previous reports the resistant varieties Loran Blood and Moscow developed acute disease symptoms after inoculation with HP and HF and these were associated with a high level of tylosis rather than mycelial growth. Tylosis and disease severity but not mycelial growth increased with higher levels of inoculum. The results suggested that susceptibility to Verticillium wilt was a complex response depending on host cultivar, fungal strain and the initial inoculum concentration. In some cultivar-pathogen combinations susceptibility was directly proportional to the amount of mycelium present in the vessels, while in others a physiological resistance mechanism independent of the degree of colonization appeared to operate. In a third category, increased disease development rather than resistance was associated with high levels of tylosis.     

An arbuscular mycorrhizal inoculum enhances root proliferation in, but not nitrogen capture from, nutrient-rich patches in soil

Most work on root proliferation to a localized nutrient supply has ignored the possible role of mycorrhizal fungi, despite their key role in nutrient acquisition. Interactions between roots of Plantago lanceolata , an added arbuscular mycorrhiza (AM) inoculum and nitrogen capture from an organic patch ( Lolium perenne shoot material) dual-labelled with ¹⁵N and ¹³C were investigated, to determine whether root proliferation and nitrogen (N) capture was affected by the presence of AM fungi. Decomposition of the organic patch in the presence and absence of roots peaked in all treatments at day 3, as shown by the amounts of ¹³CO₂ detected in the soil atmosphere. Plant N concentrations were higher in the treatments with added inoculum 10 d after patch addition, but thereafter did not differ among treatments. Plant phosphorus concentrations at the end of the experiment were depressed by the addition of the organic residue in the absence of mycorrhizal inoculum. Although uninoculated plants were also colonized by mycorrhizal fungi, colonization was enhanced at all times by the added inoculum. Addition of the AM inoculum increased root production, observed in situ by the use of minirhizotron tubes, most pronouncedly within the organic patch zone. Patch N capture by the end of the experiment was c . 7.5% and was not significantly different as a result of adding an AM inoculum. Furthermore, no ¹³C enrichments were detected in the plant material in any of the treatments showing that intact organic compounds were not taken up. Thus, although the added AM fungal inoculum benefited P. lanceolata seedlings in terms of P concentrations of tissues it did not increase total N capture or affect the form in which N was captured by P. lanceolata roots.     

Effect of arbuscular mycorrhizal fungi and Pseudomonas fluorescens on root-rot and wilt,growth and yield of Coleus forskohlii

Root-rot and wilt caused by Fusarium chlamydosporum affects the cultivation of Coleus forskohlii, a medicinal plant grown for its roots, which contain a pharmaceutically important compound called forskolin. In this study, management of this disease under low and high inoculum levels was assessed with four arbuscular mycorrhizal (AM) fungi and a strain of Pseudomonas fluorescens. The AM fungus Glomus fasciculatum and P. fluorescens were the most effective treatments that reduced the severity of root-rot and wilt of C. forskohlii by 56–65% and 61–66%, respectively, under lower and higher levels of pathogen F. chlamydosporum. G. fasciculatum increased the dry shoot and root weight by 108–241% and 92–204%, respectively, while in plants treated with P. fluorescens, an increase of 97–223% and 97–172% in dry shoot and root weight, respectively, was observed. Although P. fluorescens was effective, it gave higher root yields only under lower inoculum level of the pathogen. G. fasciculatum performed equally well under both lower and higher inoculum levels. Increase in yields with both the biocontrol agents was accompanied by increase in P uptake (230–303%) and in K uptake (270–335%). The forskolin content of the roots was significantly increased (14–21%) by G. fasciculatum, P. fluorescens or G. mosseae under lower inoculum level of pathogen.     

Management of jute yellow mosaic virus disease through cultural practices

A field experiment was conducted to investigate the effect of some management practices to minimise jute yellow mosaic virus disease. The management practices were employed at natural condition and placed randomly with four replications. The treatments were spraying malathion 57 EC, rouging and field sanitation, top dressing of nitrogenous fertiliser, mulching with straw and untreated control. The highest percentage of mosaic incidence was recorded in control and the lowest incidence was recorded in top dressing of nitrogenous fertiliser. Among the treatments, top dressing of nitrogenous fertiliser showed the best performance in terms of increasing yield (3.05?t/ha). The second highest was obtained in rouging and field sanitation which was statistically similar to spraying malathion 57 EC. The best gross margin ($379.02/ha) and increase of gross margin (63.00%) compared to control were achieved in top dressing of nitrogenous fertiliser with the highest benefit-cost ratio (4.84). However, the treatments were found significantly profitable compared to the control indicating the usefulness of the cultural practices in integrated disease management programme for healthy and profitable jute cultivation.     

Meloidogyne incognita Inoculum Source Affects Host Suitability and Growth of Yellow Nutsedge and Chile Pepper

Meloidogyne incognita (Mi) reproduction and host plant responses in chile pepper (Capsicum annuum) and yellow nutsedge (Cyperus esculentus = YNS) to three sources of inoculum obtained by rearing a single Mi population on chile, YNS, and tomato were evaluated in two factorial greenhouse experiments. The interactive effects of Mi inoculum source and crop-weed competition were determined. In the absence of YNS competition, chile growth was reduced less by Mi inoculum from chile than by inoculum from YNS or tomato. When YNS was present, chile root weight was not affected and shoot weight increased with Mi initial inoculation, regardless of inoculum source. Chile plants inoculated with Mi from tomato exhibited double the nematode reproduction observed with inoculum from chile or YNS. With chile present, Mi reproduction on YNS was nearly three times greater with inoculum from tomato, but reproduction was similar among inoculum sources when chile was absent. Reductions in YNS root mass due to competition from chile failed to reduce the total number of Mi eggs produced on YNS plants. Differences in total Mi reproduction among inoculum sources were not attributable to differences in root growth or plant competition. This study illustrates the influence of Mi-YNS interactions and previous hosts on severity of Mi infection.     

Effect of temperature,cultivars, injury of root and inoculums load of Ralstonia solanacearum to cause bacterial wilt of tomato

Bacterial wilt, caused by Ralstonia solanacearum , is responsible for severe losses in tomato crops in the world. In the present study, the effect of temperature, cultivars of tomato, injury of root system and inoculums load of R. solanacearum to cause bacterial wilt disease under control conditions was undertaken. Three strains UTT-25, HPT-3 and JHT-5 of R. solanacearum were grown at 5–40?°C in vitro to study, the effect of temperature on the growth of bacteria and maximum growth was found at 30?°C after 72?h in all the strains. Twenty-one days old seedlings of two cultivars of tomato i.e. N-5 (moderately resistant) and Pusa Ruby (highly susceptible) were transplanted into the pots and inoculated with R. solanacearum strain UTT-25 (5 × 10⁸?cfu/ml), mechanically injured and uninjured roots of the plant. The plants were allowed to grow at 20, 25, 30 and 35?°C at National Phytotron Facility, IARI, New Delhi to study the effect of temperature on intensity of bacterial wilt disease. Maximum wilt disease intensity was found 98.73 and 95.9 % in injured roots of Pusa Ruby and N-5 cultivars of tomato at 35?°C on 11th days of inoculation, respectively. However, no wilt disease was observed in both the cultivars at 20?°C up to 60?days. For detection of R. solanacearum from asymptomatic tomato plants, hrpB-based sequence primers (Hrp_rs2F and Hrp_rs2R) amplified at 323?bp was used in bio-PCR to detect R. solanacearum from crown, mid part of stem and upper parts of the plant. Another experiment was conducted to find out the inoculum potential of R. solanacearum strain UTT-25 to cause bacterial wilt in susceptible cultivar Pusa Ruby. The bacteria were inoculated at concentration of bacterial suspension 10 to 10¹⁰?cfu/ml in injured and uninjured roots of the plants separately and injured root accelerated wilt incidence and able to cause wilt disease 63.3% by 100?cfu/ml of R. solanacearum, while no disease appeared at 10?cfu/ml on the 11th day of inoculation in injured and uninjured roots of the plant.     

Integrating cultural control methods for tomato late blight (Phytophthora infestans) in Uganda

Cultural control measures against tomato late blight (Phytophthora infestans) were evaluated in six field experiments over 3 years in Uganda. Each experiment included sanitation (removal of diseased plant tissues), fungicide (mancozeb) application, and an untreated control, as standard treatments. Late blight incidence and severity were greatly reduced by sanitation, without reducing the number of healthy leaves; however, tomato growth and production were adversely affected. Fungicide treated plants retained the highest numbers of flowers and attached fruits and gave the highest yields. Three cultural practices were evaluated in repeated experiments for their effectiveness in alleviating the adverse effects of sanitation. Tomatoes grown within plastic shelters early in the production cycle were taller, and had more healthy leaves than those grown late. The numbers of diseased leaves and disease severity were equally low in sanitation alone and plastic shelter/with sanitation treatments. Flower and fruit production were significantly higher when tomatoes were grown under early shelters with sanitation than with sanitation alone. Planting density was increased without significant effects on late blight and tomato growth and production. Intercropping tomato with soybean (Glycine max) or sesame (Sesamum indicum), with sanitation, limited late blight development, but taller intercrops suppressed tomato growth and production. Integrated treatments (combining plastic shelters, a sesame intercrop and high tomato planting density) were evaluated, with and without sanitation, against the fungicide mancozeb. The mean numbers of healthy leaves in the integrated treatments were not significantly less than with fungicide treatment. Late blight incidence and severity were higher in the integrated plots without than with sanitation. The numbers of flowers and attached fruits were not significantly less in integrated treatments than in fungicide treated plots, but tomato yield was highest with fungicide treatment.     

Effects of Drip Irrigation on Population of Verticillium dahliae in Olive Orchards

Verticillium wilt of olive (Olea europaea L.), caused by Verticillium dahliae, is nowadays the most serious olive disease in Spain. The disease increments are being observed particularly in young olive plantations, favoured by several factors including inadequate cultural practices and crop production intensification, such as irrigation. Thus, three olive orchards affected by Verticillium wilt, with disease incidence ranging 30–50%, were selected to determine if the drip irrigation could favour the increase of pathogen in soil. Pathogen in soil was quantified in wet zones around the drippers and in dry zones out of them. Inoculum density in all experiments was higher in wet than in dry areas. After 4 months of watering, soil pathogen population increased considerably in wet and dry areas but inoculum density remained higher in the wet soil.     

WILT OF LUCERNE CAUSED BY SPECIES OF VERTICILLIUM

The incidence and symptoms of a disease of lucerne induced by Verticillium spp. are much less severe immediately after the crop has been first cut than later when the crop has matured. Infection becomes more marked as the season advances and the age of the ley increases, and cuts taken in the third harvest year are proving unprofitable.
In cross-inoculation studies the causal organisms of lucerne wilt, V. albo-atrum and V. dahliae prove to be only mild pathogens to hop, potato, tomato and antirrhinum, and are not pathogenic to sainfoin and clover. The last two may therefore be cultivated as fodder crops in those areas most severely affected by lucerne wilt, but the cultivation of potatoes after lucerne should be discouraged, particularly if lucerne is to be sown in future years, since the potatoes might build up a reservoir of inoculum of the lucerne pathogen. In studies of host-parasite relations it is shown that V. albo-atrum and V. dahliae from lucerne, V. albo-atrum from hop, V. tricorpus from tomato and V. nigrescens and V. nubilum from potato vary in their effects upon lucerne seedlings growing on agar media, V. albo-atrum from lucerne being, as in the field, the most virulent and the only one to reach the xylem vessels under the conditions of the experiments.
The dressings Agrosan-GN (containing mercury) and Fernasan (containing thiram) are toxic both to spores of Verticillium on lucerne seed coats and to infected plant material carried with seed, and although they are also toxic in vitro to Rhizobium mellelotus this did not prevent dressed seeds which had been inoculated with the bacterium from developing into healthy seedlings bearing normal root-nodules.     

Application of Pantoea agglomerans strain Z01 to control Fusarium wilt and its effect on the quality parameters of rockets

An antagonistic bacterium, denoted as strain Z01, was isolated from suppressive soils and identified using fatty acid profile analysis and molecular tools. Its efficacy to control Fusarium oxysporum f. sp. conglutinans on rockets (cv. Eruca sativa) was investigated in the presence of the pathogen, and its effects on the growth and pigment contents of rockets, including the plant biomass and chlorophyll and carotenoid contents, were evaluated in the absence of the pathogen. Z01 was identified as Pantoea agglomerans (GenBank accession number is JX257179). Rockets were grown under greenhouse conditions for 21 days at 25 °C and 95 % relative humidity. When the antagonist suspension was applied at 10⁸, 10⁷ and 10⁶ cells/ml by root-dipping for 20 min before transplanting, the disease incidence was 23.5 %, 28.5 % and 60.0 %, respectively. The disease incidence of rockets treated with carbendazim was 22.5 %, and in the inoculated control, the disease incidence was 82.0 %. In the absence of the pathogen, compared with the average plant biomass (10.7 g/4 plants) of the uninoculated control, application of the antagonist at 10⁸ or 10⁷ cells/ml significantly increased the biomass of the rockets by 40.9 % and 39.1 %, respectively, suggesting that Z01 had an additional ability to promote plant growth beyond its role in Fusarium wilt control. This study indicated that P. agglomerans Z01 has a potential to control Fusarium wilt in rockets. This report is the first to describe the application of P. agglomerans to control Fusarium wilt.     

Some effects, especially on yield, of artificially defoliating sugar beet

The effects of completely defoliating sugar beet at different dates from May to October were examined in four years. In each year there were plots given the usual nitrogen fertiliser application to the seedbed, and also in two of the years plots given no nitrogen. At harvest in mid-November, minimum root weights followed defoliation in July or August, but defoliation in August or later gave minimum sugar contents. When nitrogen was applied to the seedbed sugar yields were smallest after August defoliation; in the absence of nitrogen, July defoliation gave the lowest sugar yields, root yield being smaller but sugar content usually greater than with nitrogen. Up to 40% of the sugar yield was lost by July or August defoliation and late defoliation increased some of the impurities in the root juice. Yields, and recovery from defoliation, were greater with nitrogen than without. Partial defoliation in May had relatively little effect on yield. Defoliation affected the incidence of virus yellows differently in different years.     

The influence of mixed cropping on the control of potato bacterial wilt (Pseudomonas solanacearum)

Experiments with bacterial wilt (Pseudomonas solanacearum) race 3 showed that the practice of intercropping potato with maize or haricot beans markedly reduced the incidence and rate of disease development in the potato crop. This reduction in disease was considered to be an effect of the increased distances between individual potato plants, their spatial arrangement and the presence between potato plants of root systems of other plant species, all of which resulted in a reduction in plant-to-plant transmission, via the roots. The lower potato plant population associated with intercropping resulted in a slower rate of inoculum build-up in the soil and the presence of an intercrop further markedly reduced the inoculum build-up. Where farmers retain tubers for seed, but where roguing of diseased plants is not practised, the isolation of plants through intercropping was considered to facilitate an efficient selection of healthy tubers.