Reduced severity and impact of Fusarium wilt on strawberry by manipulation of soil pH, soil organic amendments and crop rotation

Strawberry (Fragaria?×?ananassa) is one of the most important berry crops worldwide. Fusarium wilt poses a serious threat to commercial strawberry production worldwide and causes severe economic losses. Our previous surveys suggested that soil pH, soil amendment with organic matter and/or crop rotation could offer opportunities for improved management of strawberry disease. Studies were conducted for the first time to determine the effects of soil pH, soil amendments with manure compost and crop residue, and crop rotation on the severity and impact of Fusarium wilt on strawberry. At soil pH 6.7, plants showed the least severe disease and the lowest reductions in shoot and root dry weight (DW) of plants from disease, significantly lower than those of plants in acidic soil at pH 5.2 or 5.8. In soil amendment with manure compost at 5.0?%, plants showed the least severe disease and the lowest reductions in shoot and root DW of plants from disease, significantly lower than those of plants in the other three levels of manure compost. In soil amendment with crop residue at 2.5?% or 5.0?%, shoot and root disease of plants and reductions in shoot and root DW of plants from disease were significantly lower than those of plants in soil without crop residue or excessive crop residue amendment at 10.0?%. Plants in soil rotated with tomato not only showed the least severe disease but also showed the lowest reductions in shoot and root DW of plants from disease, significantly lower than those of plants in soil continuously planted with strawberry without rotation or rotated with capsicum. Soil pH, soil amendment with manure compost or crop residue, and crop rotation, all significantly reduced the severity and impact of Fusarium wilt on strawberry. There is great potential for manipulating soil pH, adding soil organic amendments and utilizing crop rotation, not only to successfully manage Fusarium wilt on strawberry, but to do so in a sustainable way without current reliance upon chemical fumigants.     

Adaptation of soil solarization to the integrated management of soilborne pests of tomato under humid conditions

ABSTRACT Soil solarization was shown to be cost effective, compatible with other pest management tactics, readily integrated into standard production systems, and a valid alternative to preplant fumigation with methyl bromide under the tested conditions. Solarization using clear, photoselective, or gas-impermeable plastic was evaluated in combination with metham sodium, 1,3-dichloropropene + chloropicrin, methyl bromide + chloropicrin, pebulate, or cabbage residue. Strip solarization, applied to 20-cm-high, 0.9-m-wide beds, was conducted to achieve compatibility with standard production practices and resulted in soil temperatures 2 to 4 degrees C above those temperatures resulting when using conventional flatbed solarization. Soil temperatures were 1 to 2 degrees C higher at the edges of the raised beds, eliminating any border effects associated with solarization. Following a 40- to 55-day solarization period, the plastic was painted white and used as a production mulch for a subsequent tomato crop. The incidence of Southern blight and the density of Paratrichodorus minor and Criconemella spp. were lower (P < 0.05) in solarized plots. No differences (P < 0.05) in the incidence of Fusarium wilt and the density of nutsedge and Helicotylenchus spp. were observed between plots receiving solarization and plots fumigated with a mixture of methyl bromide + chloropicrin. The severity of root galling was lower (P < 0.05) when soil solarization was combined with 1,3-dichloropropene + chloropicrin (16.2 + 3.4 g/m(2)) and a gas-impermeable film. The incidence of bacterial wilt was not affected by soil treatments. Marketable yields in plots using various combinations of soil solarization and other tactics were similar (P < 0.05) to yields obtained in plots fumigated with methyl bromide + chloropicrin. The results were validated in several large scale field experiments conducted by commercial growers.     

Factors affecting the incidence and severity of Spongospora subterranea infection and galling in potato roots

The incidence and severity of root infection and root galling caused by Spongospora subterranea were assessed in potato plants (cv. Estima) grown under controlled environmental conditions. The effects of temperature, soil type, soil moisture regime and soil inoculum level on infection and root gall development were determined by molecular and visual methods at two plant growth stages. Root gall severity was scored at harvest, after which DNA was extracted from the roots and quantified in a real-time polymerase chain reaction (PCR) assay specific for S. subterranea . Root galling was severe at 17°C, with a disease score of 3·1 on a 0–4 scale, low (0·6) at 12°C, and did not occur at 9°C. The level of inoculum in soil, in the form of artificially added sporosori, had no effect on the incidence and severity of visual symptoms, with 21%, 41% and 33% incidence observed at 5, 15 and 50 sporosori g⁻¹ soil, respectively. Incidence of infection, as detected by the real-time PCR assay, was greater with increasing soil inoculum concentrations, ranging from 48% at 5 sporosori g⁻¹ to 59% (15 sporosori g⁻¹) and 73% (50 sporosori g⁻¹) of plants infected at maturity, but this effect was not statistically significant. No correlation was found between the occurrence of galls on roots and powdery scab on tubers of the same plants.     

Impact of soil chemistry,nutrient supplements,and fungicides on the health and yield of field-grown processing tomatoes

We report the results of a 3-year study in which we collected and analyzed soil and yield data from experimental plots in six commercial processing tomato fields in Yolo and Solano counties in California. Our objective was to assess the effect of soil chemistry and soil-delivered nutrients and fungicides on tomato plant health and fruit yield at harvest. Marketable yield, leaf necrosis, and fruit loss due to sunburn differed significantly between individual fields, with averages ranging from 81.2–138.5 Mg ha^?1, 32–72% leaf necrosis and 1.9 to 8.8% sunburnt fruit, respectively. Higher-yielding fields showed significantly lower levels of leaf necrosis and sunburn damage and a positive correlation with pre-plant soil parameters such as potassium concentration and cation exchange capacity (CEC). Interestingly, soil amendments of composted poultry manure or other nutrient supplements in low-CEC fields, but not high-CEC fields, resulted in higher fruit yield and less leaf necrosis. While all fields showed symptoms typical of Verticillium wilt and some fields showed symptoms of Fusarium wilt, Fusarium crown and root rot, corky root, and root knot nematode, none of our soil amendments, including chemical and biological fungicides, significantly or consistently reduced incidence or severity of these diseases. We discuss our findings in the context of premature vine decline of tomato, an emerging phenomenon in production fields in the Sacramento Valley, which is characterized by the loss of plant vigor and canopy cover at the onset of fruit ripening and for which causative agent(s) and management options in California remain elusive. (247 words).     

Suitability of weed species prevailing in Spanish vineyards as hosts for root-knot nematodes

Commercial vineyards in southern Spain were surveyed and sampled during October to December 2004 to determine the extent to which common weeds present were suitable hosts of root-knot nematodes infesting soils of those vineyards. Seven weed species commonly growing in grapevine soils in southern Spain were found infected by either Meloidogyne incognita or M. javanica: Amaranthus retroflexus (redroot pigweed), Anchusa azurea (ox-tongue), Chenopodium album (goosefoot), Erodium moschatum (musk stork’s bill), Malva rotundifolia (low mallow), Sinapis alba (white mustard), and Solanum nigrum (black nightshade). The host suitability of the weeds to root-knot nematodes was evaluated on the basis of root galling severity and nematode population densities in soil and roots. Also, the host–parasite relationship in these naturally Meloidogyne-infected weeds was examined. All the weed species in the study were considered suitable hosts for M. incognita and M. javanica because: (a) high Meloidogyne spp. populations occurred in roots and surrounding soil of the weed species; (b) the severity of root galling was high, and (c) well-established permanent feeding sites were observed in the histopathological studies of infected root tissues. In addition, this study presents the first reports of S. alba and A. azurea as hosts for M. incognita, and of E. moschatum as a new host for M. javanica, thus increasing the list of reported weed hosts for Meloidogyne spp. These results indicate that noticeable population densities of M. incognita and M. javanica can be maintained or increased in these weeds, at population levels higher than those previously reported for the same nematodes infecting grapevine roots. The weeds infesting vineyards thus represent an important source of inoculum of Meloidogyne spp., and furthermore may act as reservoirs of these nematodes which can be disseminated within or among vineyards by agricultural operations.     

Disease Development Following Infection of Tomato and Basil Foliage by Airborne Conidia of the Soilborne Pathogens Fusarium oxysporum f. sp. radicis-lycopersici and F. oxysporum f. sp. basilici

ABSTRACT Fusarium oxysporum f. sp. radicis-lycopersici, the causal agent of Fusarium crown and root rot of tomato, and F. oxysporum f. sp. basilici, the causal agent of Fusarium wilt in basil, are soilborne pathogens capable of producing conspicuous masses of macroconidia along the stem. The role of the airborne propagules in the epidemics of the disease in tomato plants was studied. In the field, airborne propagules of F. oxysporum f. sp. radicis-lycopersici were trapped with a selective medium and their prevalence was determined. Plants grown in both covered and uncovered pots, detached from the field soil, and exposed to natural aerial inoculum developed typical symptoms (82 to 87% diseased plants). The distribution of inoculum in the growth medium in the pots also indicated the occurrence of foliage infection. In greenhouse, foliage and root inoculations were carried out with both tomato and basil and their respective pathogens. Temperature and duration of high relative humidity affected rate of colonization of tomato, but not of basil, by the respective pathogens. Disease incidence in foliage-inoculated plants reached 75 to 100%. In these plants, downward movement of the pathogens from the foliage to the crown and roots was observed. Wounding enhanced pathogen invasion and establishment in the foliage-inoculated plants. The sporulation of the two pathogens on stems, aerial dissemination, and foliage infection raise the need for foliage protection in addition to soil disinfestation, in the framework of an integrated disease management program.     

Investigation of alternatives to methyl bromide for management of Meloidogyne javanica on greenhouse grown tomato

The root galling index and the densities of eggs in roots and juveniles in soil of the root-knot nematode Meloidogyne javanica (Treub) Chitwood on tomato, and the effect of these on crop yield were assessed in greenhouse experiments applying various treatments at two different sites in Crete, Greece. Tomato crops were grown for four cycles by rotating nematode-resistant (first and third spring crops) with susceptible (second and fourth autumn crops) cultivars and receiving the following treatments: (a) untreated control; (b) methyl bromide application before the first and third crops; (c) application of the fungus Pochonia chlamydosporia (Goddard) Zare, Gams & Evans before planting the first, third and fourth crops with a supplementary application three weeks after the beginning of the fourth crop; (d) application of oxamyl in both sites and fenamiphos in site 1 only at the second and fourth crops; (e) combination of treatments (c) and (d). The fungus density in soil was monitored three weeks after application and at the end of each crop, when roots were lifted. Pochonia chlamydosporia had a variable establishment and did not control the nematode. Its pathogenicity on eggs was not demonstrated, as in all cases galls were big, with all egg masses inside and protected from infection. The methyl bromide treatment significantly reduced root galling and egg production compared to other treatments in all crop cycles and the yield of the fourth crop was significantly greater. Nematicides reduced nematode densities compared with untreated controls and the fungus treatment, but they were less effective than methyl bromide and resulted in increased yield in one site only.     

Disease complex in coffee involving Meloidogyne arabicida and Fusarium oxysporum

Coffee corky-root disease, also called corchosis, was first detected in 1974 in a small area of Costa Rica where the root-knot nematode Meloidogyne arabicida is the dominant species. An epidemiological study revealed a constant association between Meloidogyne spp. and Fusarium sp. in cases of corky root. No corky root appears to have been reported in association with Meloidogyne exigua , which is the prevalent root-knot nematode on coffee in Costa Rica. Fusarium spp. are often cited as components of disease complexes in association with nematodes. Combined inoculations using M. arabicida or M. exigua with Fusarium oxysporum under controlled conditions showed that only the combination with M. arabicida produced corky-root symptoms on Coffea arabica cvs Caturra or Catuai. Fusarium oxysporum alone was nonpathogenic. Meloidogyne exigua or M. arabicida alone caused galls and reduction in shoot height, but no corky-root symptoms. When cultivars susceptible and resistant to M. arabicida were studied under field conditions for 5 years, all the susceptible cultivars exhibited corky-root symptoms on 40–80% of their root systems. Cultivars that were resistant to M. arabicida but not to M. exigua showed no corky root. These observations lead to the conclusion that corky-root disease has a complex etiology, and emphasize the dominant role of M. arabicida as a predisposing agent to subsequent invasion by F. oxysporum . Consequently, genetic resistance to M. arabicida appears to provide an effective strategy against the disease.     

Interactions between Fusarium oxysporum f. sp. racheiphilum and Meloidogyne spp. in Vigna unguculata. 2. Specificity of different taxa

A California isolate of Meloidogyne javanica increased Fusarium wilt symptoms in cowpea cultivars California Blackeye No. 3 (CB3) (resistant to wilt) and Grant (tolerant) inoculated with each of the three races of Fusarium oxysporum f. sp. tracheiphilum. The same isolate of M. javancia did not similarly increase wilt in wilt-resistant cultivar CB7977 inoculated with two isolates of race 3 of F. o. tracheiphilum. Six of seven isolates of M. javanica caused similar increases in vascular discoloration in cultivar CB3. but one isolate of M. javanica and seven of M. incognita did not. Vascular discoloration rating was positively correlated with galling severity. However, increasing the initial inoculum density, and thus galling index, of one isolate of M. incognita did not increase vascular discoloration. The vascular discoloration ratings for the wilt-susceptible CB5 controls in each experiment were higher than those for the wilt-resistant cultivars infected with M. javanica. It is hypothesized that M. javanica but not M. incognita reduces, but does not eliminate, resistance to all races of F. o. tracheiphilum in cultivars CB3 and Grant.     

Incidence of soil-borne plant pathogens isolated from barley and winter wheat, and other crops in the rotation, on Prince Edward Island

Barley and winter wheat were sampled over 3 years to characterize soil-borne organisms involved with cereal crown and root disease complexes. Winter wheat crowns were infected more often by Fusarium avenaceum and F. sambucinum than barley crowns. Bipolaris sorokiniana and F. graminearum were more common in crown tissue of barley underseeded with clover than in winter wheat crowns. In roots, the incidence of F. avenaceum was highest in winter wheat. In soil, populations of Rhizoctonia solani and F. sambucinum were higher in winter wheat than barley underseeded with ryegrass. Stunt nematodes (Tylenchorhynchus spp.) were greatest in winter wheat and barley underseeded with ryegrass. The incidence of F. avenaceum in roots of winter wheat correlated positively with the severity of crown and root rot symptoms. The severity of barley crown and root rot correlated positively with the incidence of R. solani in crowns, F. avenaceum in crowns and roots, B. sorokiniana in roots, and numbers of stunt nematodes in the soil. The incidence of soil-borne organisms was also recorded in annual ryegrass. soybean, potato, pea, and clover which were grown in rotation with barley and winter wheat. The incidence of R. solani and F. avenaceum was highest in clover crown tissue, and F. sambucinum was prevalent in soybean hypocotyls. Soil population levels of R. solani, F. avenaceum , and stunt nematodes were greatest in ryegrass and pea soil just prior to ploughing down or harvesting these crops, respectively.     

Interactions between Fusarium oxysporum f. sp. tracheiphilum and Meloidogyne spp. in Vigna unguiculata. 1. Effects of different inoculum densities on Fusarium wilt

The effects of chlamydosporesandconidia of Fusarium oxysporum f sp. tracheiphilum at different initial spore concentrations were compared in the wilt-susceptible cowpea cultivar California Blackeye No. 5 (CB5). In glasshouse experiments with one inoculum density of either Meloidogyne incognita or M javanica, chlamydospores resulted in greater incidence and severity of Fusarium wilt than conidia at the same inoculum densities. Wilt symptoms also increased on wilt-resistant cultivar CB3 as inoculum densities of M. javanica were increased. When three cultivars were infested with moderate or high densities of both F. o. tracheiphilum and M. javanica. only CB5 developed sere wilt at either inoculum density. The wilt-tolerant cultivar Grant had mild wilt symptoms in most plants at moderate inoculum densities, and a tenfold increase in inoculum did not increase wilt ratings. CB3. however, had higher incidence and severity of Fusarium wilt symptoms at high inoculum densities, although 60% of the plants survived for 9 weeks.     

Soil solarization as a substitute for methyl bromide fumigation in greenhouse tomato production in Cyprus

Preplant soil fumigation with methyl bromide (MB) is presently standard practice in greenhouse tomato production. Since this compound is scheduled to be phased out by 2005, the possibility of using solarization as an alternative soil disinfestation method was examined in four greenhouse tomato trials. Solarization was applied for 8 weeks in July-August, using transparent polyethylene sheets for soil mulching, and compared with MB fumigation applied in September, before planting, at 80 g/m². Solarization raised the maximum soil temperature by 9°C and reduced the population density ofFusarium spp. in soil by 91–98%. Similar reductions of soil inoculum (95–99%) were obtained with MB fumigation. Both methods provided effective control of Fusarium wilt, Verticillium wilt and corky root rot on tomato plants. MB fumigation was in addition highly effective against root-knot nematodes, whereas nematode control with solarization did not exceed 50%. Both treatments resulted in similar fruit yield increases, ranging within 90–140% compared with plants grown in untreated soil. During the second cropping season following soil treatment, solarization exhibited two times higher residual effectiveness against vascular wilt diseases compared with MB fumigation. The latter treatment, however, was superior to solarization in its residual effectiveness against root-knot nematodes and to a lesser extent against corky root rot. Fruit yields from solarized and MB-fumigated soil during the second cropping season were higher than those obtained from untreated soil by approximately 35% and 60%, respectively. In Cyprus, solarization appears to be an effective alternative to MB fumigation in greenhouse tomato production, especially if integrated with other approaches enabling more effective nematode control.     

Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae,the cause of fusarium wilt of lettuce

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non‐susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7·4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt‐resistant lettuce cultivars. Thus, disease‐resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above‐ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.     

重庆市草坪草病害种类调查及病原鉴定初报

为了明确重庆草坪草主要病害及其病原菌的种类,作者调查了重庆草坪草病害种类,并分离鉴定了病原。结果表明：重庆草坪草常见病害有锈病、结缕草炭疽病、褐斑病、镰刀枯萎病、结缕草弯孢霉叶枯病和结缕草根结线虫等病害,分别由柄锈菌（Puccinia spp.）、尾状炭疽菌（Colletotrichum caudatum）、立枯丝核菌（Rhizoctonia solani）、镰刀菌（Fusarium sp.）、弯孢霉（Curvularia spp.）和根结线虫（Meloidogyne sp.）危害所致。     

Use of crop residues for the control of Meloidogyne incognita under laboratory conditions

This laboratory study evaluates the biofumigant effect of different organic materials with the aim of developing non-chemical alternatives for the management of Meloidogyne incognita (Kofoid & White) Chitwood populations. Sources of organic material from the production system were selected with the aim of reducing agricultural residue accumulation problems as well as decreasing the costs due to the use of chemical fertilizers and pesticides. The selected materials were residues from pepper, strawberry, tomato and cucumber crops, orange juice industry residues, commercial manure and sheep manure, applied at different dosages. Two biofumigation assays were performed under laboratory conditions, using alkaline soils from the Torreblanca area (Murcia, Spain) and acidic soils from the Villa del Prado area (Madrid, Spain). The assays evaluated the effect of the treatments on M. incognita juveniles and other soil organisms, the nematode galling index on tomato roots (susceptible cv. Marmande) grown in the biofumigated soil and soil fertility parameters. The results showed that all biofumigant materials significantly decreased M. incognita populations and galling indices in tomato cv. Marmande. A greater effect was observed on galling indices when applying crop residues together with manure than with the residues alone. Biofumigation had a general beneficial effect on soil fertility, generally increasing nitrogen, organic carbon, pH and potassium levels, and also calcium levels when crop residues of pepper and strawberry were applied. There were no important variations in the number of saprophagous nematodes, dorylaimids and enchytraeids.     

Integrated management of Meloidogyne incognita‐Fusarium oxysporum f. sp. ciceri wilt disease complex in chickpea

A field experiment was conducted to study the management of a root‐knot nematode, Meloidogyne incognita (Kofoid and White) Chitw. (Tylenchida: Heteroderidae)‐wilt inducing fungi, Fusarium oxysporum Schlecht. emend. Snyd & Hans. f. sp. ciceri (Padwick) Snyd. & Hans. (Moniliales: Tuberculariaceae) wilt disease complex in chickpea (Cicer arietinum L.). cv. Annegiri. The results indicated that integration of soil solarization (for 6 weeks), VA mycorrhizal fungus (VAM), Glomus fasciculatum inoculation (12g/hill) and seed treatment with carbosulfan (3% w/w) was highly effective in reducing population levels of both pathogens, root‐knotdisease and wilt incidence and in increasing chickpea grain yield significantly. However, seed treatment with carbendazim (0.25% w/w) together with carbosulfan (3% w/w) was not only effective in reducing the wilt disease complex but also economic with an incremental cost: benefit ratio of 1: 2.4.     

Interactions Between Meloidogyne artiellia, the Cereal and Legume Root-Knot Nematode, and Fusarium oxysporum f. sp. ciceris Race 5 in Chickpea

ABSTRACT In the Mediterranean Basin, Fusarium oxysporum f. sp. ciceris and the root-knot nematode Meloidogyne artiellia coinfect chickpea. The influence of root infection (after inoculation with 20 nematode eggs and second-stage juveniles per gram of soil) by two M. artiellia populations, from Italy and Syria, on the reaction of chickpea lines and cultivars with partial resistance to Fusarium wilt (CA 252.10.1.OM, CA 255.2.5.0, CPS 1, and PV 61) and with complete resistance to F. oxysporum f. sp. ciceris race 5 (CA 334.20.4, CA 336.14.3.0, ICC 14216 K, and UC 27) was investigated under controlled conditions. In genotypes with partial resistance, infection by M. artiellia significantly increased the severity of Fusarium wilt, irrespective of the fungal inoculum density (3,000 or 30,000 chlamydospores per gram of soil), except in cultivar CPS 1 at the lower fungal inoculum density. In genotypes with complete resistance to Fusarium wilt, infection by M. artiellia overcame the resistance to F. oxysporum f. sp. ciceris race 5 in CA 334.20.4 and CA 336.14.3.0 but not in ICC 14216 K, irrespective of the fungal inoculum density, and overcame the resistance in UC 27 only at the higher inoculum density. Infection by the nematode significantly increased the number of propagules of F. oxysporum f. sp. ciceris race 5 in root tissues of genotypes with complete resistance to Fusarium wilt, compared with roots that were not inoculated with the nematode, irrespective of the fungal inoculum density, except in ICC 14216 K, in which this effect occurred only at the higher inoculum density. Reproduction of an M. artiellia population from Syria in the absence of F. oxysporum f. sp. ciceris race 5 was significantly higher than that of a population from Italy in all tested chick-pea genotypes except ICC 14216 K. However, there was no significant difference between the reproduction rates of the two nematode populations in plants infected with F. oxysporum f. sp. ciceris race 5, irrespective of the fungal inoculum density and the reaction of the genotypes to the fungus.     

Excised tomato root culture as a tool for testing root-knot nematode resistance

The excised root culture inoculation technique proved to be an accurate method for evaluating resistance to root-knot nematodes in tomato (Lycopersicon esculentum). Root cultures of seven tomato cultivars growing in an incubator at 25°C were inoculated with egg masses ofMeloidogyne javanica. Infection severity was determined 5 weeks after inoculation. Nematode-susceptible cultivars were characterized by high galling indices and resistant cultivars by low ones. The excised root culture technique is simple, allows results to be obtained rapidly, and no greenhouse space is needed.     

Fusarium species from the cassava root rot complex in west Africa

ABSTRACT Fusarium species are a significant component of the set of fungi associated with cassava root rot. Yield losses due to root rot average 0.5 to 1 ton/ha but losses >3 ton/ha, an equivalent of 15 to 20% yield, often occur. This paper reviews previous work on cassava root rot and summarizes a few recent studies on Fusarium species associated with the disease. Our studies in Cameroon showed that 30% of rotted tubers were infected by Fusarium spp. 12 months after planting and represented 25% of all the fungal isolates recovered. Other commonly recovered fungi were Botryodiplodia theobromae and Armillaria spp. Numerous and diverse species of Fusarium were associated with rotted cassava roots in Nigeria and Cameroon. At least 13 distinct amplified fragment length polymorphism (AFLP) groups of Fusarium were distinguishable, each group probably a distinct species, and many of them might represent previously undescribed Fusarium species. The two largest of the AFLP groups correspond to F. oxysporum and F. solani species complex. The distribution of Fusarium spp. varied among countries and among locations within a country, suggesting that germ plasm resistant at one location may not be resistant at another. Fusarium spp. also cause seedling blight of cassava and can be recovered from the stems of infected plants up to 1 m above the ground. Therefore, the pathogen can spread with stems cut as planting material. Fusarium spp. also can colonize Chromolaena odorata, the dominant weed in short fallows, which could further complicate management efforts by serving as an alternative host for strains that colonize cassava.     

Somaclonal selection in potato for resistance to common scab provides concurrent resistance to powdery scab

Variant somaclones of potato cultivar Russet Burbank, selected for resistance to common scab using in vitro cell selection techniques, were tested for resistance to powdery scab, another important disease of potato caused by Spongospora subterranea. This pathogen also invades roots, producing root galls. Most variants consistently showed increased resistance to powdery scab, both in field and glasshouse challenge, when compared to the parental cultivar, several significantly so. On average, the best variant reduced powdery scab incidence by 51% and severity (tuber surface coverage) by 64%. In contrast, no improvement in the extent of root infection and root galling was seen. These results suggest host interactions during root and tuber infection are distinct. Correlation analyses of disease indices amongst the selected variants showed no association between Sp. subterranea root infection and gall scores, nor between root infection and tuber disease severity. However, a weak positive association was found between root gall score and tuber disease, and a strong correlation between tuber disease incidence and severity scores. Interestingly, positive correlations were also found between the extent of powdery and common scab resistance expressed and both incidence and severity of these diseases within the variants, suggesting a common defence mechanism. The role of thaxtomin A in selecting for concurrent resistance to both diseases is discussed.