Effects of Soil Flooding on the Survival of Two Potato Pathogens, Sclerotinia sclerotiorum and Verticillium dahliae

Results of studies on survival of sclerotia of Sclerotinia and microsclerotia of Verticillium, demonstrated that soil flooding in western Washington is a possible alternative field rotation practice for S. sclerotiorum (white mold), but not for V. dahliae (Verticillium wilt). Cone-tainer experiments in the greenhouse showed that flooding at 16.5 °C caused S. sclerotiorium sclerotia to lose viability between 12 and 24 weeks while a growth chamber experiment revealed that flooding for 18 weeks at 11 °C or 20 °C was sufficient. V. dahliae microsclerotia appeared resistant to flooding under greenhouse and field settings; recovery ranged within 5 to 10 % of the initial soil population after 6 and 12 months. Potatoes planted into field microplots either flooded or fallowed the previous summer had similar Verticillium wilt ratings and potato yield. Lack of control of V. dahliae by flooding may be due partly to relatively low soil temperatures in a cool, marine climate.     

The Effect of Alfalfa Residue Incorporation on Soil Bacterial Communities and the Quantity of <Emphasis Type="Italic">Verticillium dahliae</Emphasis> Microsclerotia in Potato Fields in the Columbia Basin of Washington State,USA

Verticillium wilt, caused by the soil-borne fungus Verticillium dahliae, is one of the most important diseases of potato in North America. Soil incorporation of alfalfa residues prior to planting potato could be a nonchemical Verticillium wilt management tactic by reducing the number of viable microsclerotia in field soil. Verticillium dahliae microsclerotia were quantified in field soils where organic material from alfalfa was incorporated, and numbers of microsclerotia were compared to fields where alfalfa residue was not incorporated. In addition, bacterial metagenomics was utilized to characterize soils where organic material from alfalfa was or was not incorporated to determine if alfalfa residue incorporation facilitates the formation of soils that suppress or kill V. dahliae microsclerotia. The number of V. dahliae microsclerotia in soil was greater (P = 0.0003) in fields where crop residue was incorporated than fields without incorporation when chloropicrin was used as a fumigant. Conversely, the number of V. dahliae microsclerotia observed in potato plants did not differ (P = 0.4020) between fields where residues were or were not incorporated if chloropicrin was used. Alfalfa residue incorporation did not significantly alter the soil bacterial metagenome compared to fields not subject to residue incorporation in both years of study. Despite these conclusions, the method can be employed to analyze the effect of grower practices with the intent of linking a field practice to increasing soil bacterial diversity and decreasing Verticillium wilt severity on potato.     

Seasonal variation and effects of wheat rotation on populations ofVerticillium dahliae Kleb. in Ohio potato field soils

Soil population levels ofVerticillium dahliae in Ohio were monitored from May–October, 1982–1985, in 15 fields in potato-wheat rotation and two fields in potato monoculture. Population levels in fields in rotation ranged from 0 to 86 microsclerotia/10 g of air-dried soil with average values during each 6-month sampling period of 9.7, 12.9, 9.6 and 19.6 microsclerotia/10 g of soil for 1982, 1983, 1984 and 1985, respectively. In at least 2/3 of the fields sampled each year, soil populations ofV. dahliae peaked in either July or August, regardless of whether fields were under wheat rotation or potato monoculture. Of the 15 fields under potato-wheat rotation, 13 showed a general pattern of increased populations ofV. dahliae in one or both years following cropping to potato. Among those 13, significantly higher populations (P < 0.05) were observed in one wheat field in 1983 and in eight fields in 1985. In the two fields in potato monoculture, one consistently had population levels ofV. dahliae 3–4 times higher than any other field sampled in this study. Factors that may contribute to periodic changes inV. dahliae populations, implications of these changes in interpreting soil population data, and usefulness of a potato-wheat rotation in managingV. dahliae populations in Ohio are discussed.     

Induction of soil suppressiveness to verticillium wilt of potato by successive croppings

Nine soils collected in New Jersey and Maryland were infested with microsclerotia ofVerticillium dahliae and planted with potato cv. Superior for five successive crops in the greenhouse to induce suppressiveness to Verticillium wilt. Potatoes were harvested 9–12 wk after planting and disease incidence was determined by recovery ofV. dahliae from stem segments on selective medium. Six of the nine soils had high disease incidence during the first cropping and remained conducive to Verticillium wilt throughout the experiment. Disease incidence increased in soils 3 and 9 over time, while disease incidence decreased nonlinearly in soil 1. Viability of microsclerotia buried in soil 9 for 4 wks was reduced compared to the viability of microsclerotia buried in soils 1, 3, and 6. Addition of 50 or 100 g hydrated lime/10 kg soil to soil 9 eliminated its suppressiveness to Verticillium wilt. Repeated cropping of the same soil can induce suppressiveness to Verticillium wilt. Soils exhibiting induced suppressiveness may be a source of new antagonists againstV. dahliae.     

Fertility and management practices to control verticillium wilt of the russet burbank potato

Management practices for the suppression of Verticillium wilt of Russet Burbank potato include sanitation, use of optimum sprinkler-irrigation practices, soil solarization, and an adequate soil fertility program. Among all cultural factors considered, nitrogen (N) deficiency in potato tissue was most commonly associated with the severity of Verticiilium wilt in Russet Burbank potato. Field studies have shown that increased N availability suppresses Verticiilium incidence on cv Russet Burbank while having no effect on cv Norgold Russet. Studies from both greenhouse and field show that the colonization ofV dahliae in potato stem tissue is suppressed in cv Russet Burbank when the availability of Phosphorous (P) is increased to the optimum. Following one season of cropping with Russet Burbank potato, the treatment providing the optimal N availability also suppressed the increases ofV dahliae populations in the soil during the following year of consecutive cropping. Similarly, after two seasons of continuous potato cropping, treatments with optimal P had lower soil populations ofV. dahliae in soil. Results show the suppression of Verticiilium wilt with optimal fertility. Verticiilium wilt [caused by eitherVerticiilium dahliae Kleb, (microsclerotial form) orVerticiilium albo- atrum Reinke and Berthold (dark mycelial form)] is one of the most severe diseases of potato in the United States. Potato yields, tuber size, and specific gravity may be substantially reduced by this disease, depending on severity, time of occurrence, and growing season. In Idaho and other arid growing regions of the United States, Verticiilium wilt is caused byVerticillium dahliae Kleb. Idaho field studies involving cropping practices, soil fumigation treatments, solarization, and Verticillium-resistant potato clones all support the importance ofV dahliae upon potato yield. Data from these studies show that yield losses due toV. dahliae commonly approximate 5 to 12 metric T/ha (5, 6). Table 1 illustrates the effect of several cropping practices upon relativeV dahliae populations in soil with the effects upon both disease severity and potato yield. With continuous cropping of Russet Burbank,V dahliae populations in soil increased, and yields were reduced by 18 to 19 metric Tlha when compared to locations that had been allowed to remain either weed-free and fallow or where corn had been previously cropped. It was estimated thatV dahliae was influencing yield by about 10 to 12 metric Tlha based on a regression analysis of yield as a function ofV dahliae populations in soil. Similarly, whenV dahliae was suppressed by fumigation treatments involving mixtures of dichloropropene and picfume, yields more significantly increased by 6.5 to 12 metric T/ha (5). Although many factors may influence yields, clones with higher Verticillium resistance than Russet Burbank generally out-yield Russet Burbank. Figure 1 demonstrates the effect of solarization (a technique involving the laying of clear plastic on soil for the purpose of elevating soil temperatures to killV dahliae). When Verticillium was controlled in this manner (lo), there was no significant yield difference between the Russet Burbank potato and the highly resistant A68113-4 clone. The A68113-4 clone grown in non-solarized soil out-yielded the Russet Burbank in non-solarized soil by 31 percent while the solarization treatment significantly increased yield for the Russet Burbank and A68113-4 clones by 46 and 18 percent, respectively. These yield responses were observed in a field where inoculum levels ofV dahliae were relatively low (2.10 cfulgrn of soil). With these losses, the need to develop control procedures is great and to achieve this control the need also exists to accurately evaluate the effect of treatments uponV dahliae. Our Idaho studies have routinely utilized such a procedure (9) in combination with comparisons of both disease severity and yield.     

Effect of Soil Temperature,Injection Depth,and Metam Sodium Rate on the Management of Verticillium Wilt of Potato

Verticillium wilt, caused by Verticillium dahliae Kleb., is a primary component of the early dying complex of potato (Solanum tuberosum L.) in the United States. Although genetic resistance to V. dahliae exists and has been incorporated into several potato cultivars, the commercial potato industry is still dominated by cultivars susceptible to the pathogen. As a result, soil fumigation with metam sodium remains an important means by which Verticillium wilt is controlled, despite its expense and potentially negative environmental impact. Recent restrictions on metam sodium use by the Environmental Protection Agency directed at reducing exposure to vapor emissions have increased the need to improve shank injection of the soil fumigant. In studies reported here, the application of metam sodium reduced the severity of Verticillium wilt, however, soil temperature at the time of injection, metam sodium injection depth, and application rate had little overall effect. In 2011, temperature at the time of metam sodium injection did not result in significant differences in any parameter evaluated. However, in 2012, soil populations of V. dahliae, wilt severity and host colonization were significantly reduced when metam sodium was applied at 4 °C compared to 13 or 15 °C. No significant differences were observed between a single or two metam sodium injection depths in any parameter evaluated across the 2 years the study was conducted. While all rates of metam sodium significantly reduced soil populations of V. dahliae compared to the non-treated control, significant differences across rates were rarely observed. Improved control of Verticillium wilt and increased yield can be achieved as a result of these studies. The effective control of Verticillium wilt can be obtained by using metam sodium at a comparatively low rate of 373 l/ha, particularly when applied at a relatively cold soil temperature of 4 °C using a single injection depth of 25 cm. The potential impact of these application modifications of metam sodium in reducing emissions and non-target exposure is discussed.     

Soil properties affected by combinations of soil solarization and organic amendment

A field experiment was conducted to investigate the effects on soil properties of solarization combined with rice bran additive. The treatments included control (bare soil), black polyethylene film mulch (BM), clear polyethylene film mulch (CM), rice bran mixture (RBMx), rice bran mulch (RBM), rice bran mixture and black polyethylene film mulch (RBMx + BM), rice bran and black polyethylene film mulch (RBM + BM), rice bran mixture and clear polyethylene film mulch (RBMx + CM), and rice bran and clear polyethylene film mulch (RBM + CM). Initially, 15 kg m⁻² of rice bran was applied to each treatment as the organic amendment. A cover of black or clear polyethylene film was then used to begin the soil solarization process. Approximately 49 days later, the polyethylene film was removed and five broccoli seedlings per replication were planted 6 weeks later. The combination of rice bran mixture with the clear/transparent polyethylene film mulch (RBMx + CM) raised both the mean and maximum soil temperatures over 0–30 cm of soil depth. Maximum soil temperature under the RBMx + CM treatment was the highest of all treatments (77°C) and were about 32°C greater than under the control, while it was only 52°C under RBM + BM treatment. Combining rice bran with clear/transparent polyethylene film mulch during the soil solarization process is recommended for raising soil temperatures. Further investigation over a longer period of soil solarization process with rice bran–organic amendment and polyethylene film mulch combinations is required to more accurately determine their effects on soil properties and crop production.     

Potato Early Dying and Yield Responses to Compost,Green Manures,Seed Meal and Chemical Treatments

Verticillium dahliae Kleb. is a soilborne fungal pathogen of many crops. In potato, it is the major causal agent of Early Dying. In Manitoba, potato fields planted with cv. Russet Burbank are infested with highly pathogenic V. dahliae isolates, which can produce up to 90 % disease severity. The objective of the study was to evaluate selected compost, green manure, and seed-meal treatments, in comparison with the soil fumigant Vapam, for their ability to reduce propagule density of V. dahliae in soil and decrease disease, and to enhance potato yield. Select green manure crops (oriental and white mustard, Canada milk vetch, sorghum-sudangrass, rye, alfalfa, oat/pea mixture), organic amendments (composted cattle manure and mustard seed-meal), and Vapam, and crop sequences that contribute to the suppression of Verticillium, or the improvement of potato yield were used in a 3-year field study initiated in 2006. Survival in soil of microsclerotia was evaluated as a measure of treatments’ success in potentially reducing Early Dying. Compost and seed-meal treatments, compared to an untreated control, reduced incidence to 30 and 40 %, respectively, but only seed-meal reduced V. dahliae propagule density. Overall, green manures over 1 or 2-years were ineffective in reducing propagule density or improving potato yield. Vapam was partially effective in reducing the propagule density only at the beginning of the potato season, but it did not reduce disease incidence compared to the control. Compost and seed-meal are promising as alternative control of V. dahliae. Only compost reduced disease and increased potato yield, which was associated with improved nutrient availability (phosphorus and sulfate) in soil.     

Control of sclerotia and apothecia of Sclerotinia sclerotiorum by metham-sodium,methyl bromide and soil solarization

The effects of metham-sodium (MES) at 35 ml/m², methyl bromide (MB) at 50 g/m², and soil solarization (S-S) on viability of sclerotia and production of apothecia of Sclerotinia sclerotiorum, were studied at two irrigated arid sites in Israel. MES, MB and S-S killed sclerotia in the top 10 cm to a similar degree (over 90%) and consequently reduced apothecia production: over 20 times more apothecia were observed in the controls than in the treated plots.     

The Low Potential of Teff (<Emphasis Type="Italic">Eragrostis tef</Emphasis>) as an Inoculum Source for <Emphasis Type="Italic">Verticillium dahliae</Emphasis>

Teff (Eragrostis tef) is a fine stemmed annual grass and gluten free small grain that is of interest as a forage, cover, or a rotation crop. Little is known about the susceptibility of teff to many diseases. Teff could be grown in rotation with potato in the northwestern United States provided teff cultivation is economical and does not increase soil populations for pathogens affecting rotation crops such as Verticillium dahliae. Verticillium dahliae infects a wide range of dicotyledonous plants, making it one of the most important fungal pathogens of crop plants in North America, including potato. The objective of this study was to quantify the susceptibility of teff to eight V. dahliae isolates and compare the susceptibility of teff to eggplant. Teff was confirmed as a host for V. dahliae, as indicated by the presence of microsclerotia in teff stems and roots after artificial inoculation in two years of greenhouse studies. The number of microsclerotia produced in teff did not differ between mint and potato pathotypes of V. dahliae. No V. dahliae isolate produced significantly greater numbers of microsclerotia than any of the seven other isolates tested in a two-year study. Microsclerotia production of V. dahliae in teff was consistently less than in susceptible eggplant cv. Night shadow in both greenhouse experiments (P?<?0.02). It is unlikely that teff infected by V. dahliae will proliferate microsclerotia of mint or potato-aggressive pathotypes, especially when compared to susceptible eggplant cultivars.     

QuantifyingVerticillium dahliae in soils collected from potato fields using a competitive PCR assay

A quantitative PCR assay based on the competitive PCR technique was compared to the classical soil dilution (SD) method for its ability to estimateV. dahliae propagules directly in soils collected from fields under potato production. A strong correlation (r = 0.97) was observed betweenV. dahliae propagules estimated using the quantitative PCR assay and those using the SD method. Coamplification ofV. dahliae DNA with competitor DNA provided accurate quantification in the range of 10² to 10⁷ spores and 1 to 100 microsclerotia/g of soil. The number ofV. dahliae propagules detected in PEI soils ranged from 4.9 to 15.6 and 0.06 to 0.5 microsclerotia/g of soil for PCR assay and SD method, respectively. The strong correlation between PCR assay and SD method and the non significant differences between replications of PCR estimates ofV. dahliae propagules in soils (P< 0.05) show that the PCR assay is reliable and reproducible, and comparable to the SD method. This method is fast, does not depend on the subjectiveness of the traditional plating method, and offers an improvement in speed and precision over currently used methods. In addition, it can be extended to estimateV. dahliae propagules in other pathosystems and finds immediate and practical use in epidemiological studies to determine the effects of various crop management strategies on the dynamics and level of fungal propagules in the soil in order to establish threshold levels for assessing disease risks and develop disease prediction systems.     

Effect of haulm treatments on the formation of microsclerotia ofVerticillium dahliae Kleb. on potato

Summary In four pot experiments, potato plants of cv. Element were artificially infected withV. dahliae. At an early and a late harvest haulms were killed chemically, by burning or by various other treatments, including cutting them into pieces of different lengths and keeping the debris on the soil surface or covering with soil. After 4 weeks the plant material was air-dried and the number of microsclerotia per mg was determined. At the early harvest, in two experiments, the chemical treatment yielded more microsclerotia than the cutting treatments. Covering colonised haulm tissue with non-sterilised soil was effective in inhibiting microsclerotia formation. Shorter haulm pieces led to fewer microsclerotia at the later harvest if the material was kept on the soil surface. The variation in microsclerotial yield and in treatment effects among the different experiments was large.     

Fusarium proliferatum and Fusarium tricinctum as causal agents of pink rot of onion bulbs and the effect of soil solarization combined with compost amendment in controlling their infections in field

Laboratory tests and field trials were conducted in 2010 and 2011 in Salerno province, Southern Italy, to determine the causal agent(s) of pink rot of roots and bulbs of onion and the effects of soil solarization combined with municipal food waste compost amendment in reducing their infections on bulbs of onion. Three fungi were mainly isolated: Fusarium proliferatum and Fusarium tricinctum on bulbs and roots and Pyrenochaeta terrestris exclusively on roots. P. terrestris produce symptoms on roots, but it was not able to cause damage on unwounded bulbs. F. proliferatum and F. tricinctum were able to induce symptoms when inoculated separately and showed no synergistic effects on severity of symptoms on bulbs. They induced symptoms on roots at a lesser extent. Combined inoculations of P. terrestris with both Fusarium species on roots have not increased symptoms severity of pink rot of onion. This is the first report of F. tricinctum as agent of pink rot of onion bulbs. Two years field trials have been carried out from 2009 to 2011 to evaluate the efficacy of soil solarization, applied alone or in combination with municipal food waste compost amendment, for controlling pink rot of onion bulb caused by F. proliferatum and F. tricinctum. For this purpose, soil solarization, alone and combined with compost amendment, were compared with metham sodium and an untreated control. The results of both trials demonstrated that, in presence of pink rot of onion bulbs, compost amendments, applied after solarization, strongly reduced the efficacy of soil solarization from 68.7% to 16.3% in 2010 and from 76.9% to 4.6% in 2011. Organic amendments applied before soil solarization significantly reduced the efficacy of this technique only in 2011. Significant yield increases were detected at the end of both trials in plots treated with solarization alone, metham sodium and compost applied after solarization. The highest incidence of unmarketable infected plants recorded in plots solarized and then amended with compost was compensated by the increase of bulb size and plant weight. These results demonstrated as the benefits achieved with compost amendments in controlling pink rot of onion are not easily predictable and applicable on large scale.     

Thermal inactivation of Plasmodiophora brassicae Woron. and its attempted control by solarization in the Salinas Valley of California

Thermal inactivation of resting spores of P. brassicae Woron. in glasshouse soil depended on temperature, duration of treatment, inoculum concentration, and soil moisture. At 42, 44 and 50°C, the relationship between temperature and the time for thermal inactivation plotted on a semi-log scale was linear. Treatment times up to 45 days at 30°C and 37°C did not reduce infectivity. The detection threshold in the system was 10^0·5 spores/g of soil. At all temperatures tested, inactivation was achieved more rapidly in soil infested with 10² spores/g than with 10⁶ spores/g. Heat treatment was more effective in saturated soil than in half-saturated soil. Soil temperatures in the field in the northern Salinas Valley were increased 11–14°C by tarping with clear, polyethylene plastic. The average weekly maximum temperature and minimum temperature at a 10 cm depth under tarps were 38°C and 29°C respectively. Solarization reduced disease development after a 10-week treatment but not after a 5-week treatment.     

The role ofErwinia carotovora in the epidemiology of potato blackleg. II. The effect of soil temperature on disease severity

Relative disease severity (seedpiece decay or post emergence blackleg) incited byErwinia carotovora var.carotovora (Ecc) andErwinia carotovora var.atroseptica (Eca) was studied in the field and the greenhouse. When inoculated tubers were planted in cool soils in the field or greenhouse (7.0–18.5°C average minimum and 16–26°C average maximum temperature during the first 30 days after planting)Eca caused significantly more disease thanEcc. Under these experimental conditionsEcc did not cause significantly more disease than that found in non-inoculated controls. When soil temperatures were high at planting time (21.4–24.0°C average minimum and 29.6–35.0°C average maximum temperature for the first 30 days after planting), inoculation withEcc resulted in severe disease whileEca produced no more infection than that found in uninoculated controls. At intermediate soil temperatures both were infective. Seedpiece decay by both organisms was correlated with soil temperatures during the first 30 days after planting. As the soil temperature increased, inoculation withEcc resulted in more disease than inoculation withEca. At lower soil temperatures the opposite result was obtained. Field and greenhouse studies showed that bothEca andEcc were able to cause typical post emergence blackleg infection, the former at low soil temperature and the latter at high temperature conditions. The data suggest that bothEcc andEca are components of the potato blackleg and soft rot disease complex. Environmental conditions, especially soil temperature, determine which organism is causative under a particular set of conditions.     

Soil suppressiveness to Meloidogyne javanica as induced by organic amendments and solarization in greenhouse crops

We assessed soil suppressiveness against root-knot caused by Meloidogyne javanica, following the incorporation of crop residues (organic amendments [OA]) and soil solarization, under agricultural conditions. Two field experiments were established in tomato greenhouses and a third in a nethouse for growing Antirrhinums, all infested with M. javanica. Dried residues of wild rocket (WR) were incorporated into the soil and then it was solarized to moderate temperatures to partially effect the pathogen population. Root galling on the roots of the successive tomato crop was significantly reduced to different levels by WR, solarization or their combination, in two experiments. Solarization, alone or combined with WR amendment, significantly reduced root knot incidence in the roots of snapdragon, but suppression of root galling in the roots of snapdragon in the following crop was not evident. We further assessed the potential of various herb residues, incorporated in small plots, and solarization to induce soil suppressiveness against root-knot caused by M. javanica. Amending soil with residues of WR, tarragon, peppermint or sage induced soil suppressiveness to root knot even when M. javanica was introduced into the soil after the termination of the treatment, and reduced the galling index in subsequently grown tomato plants, compared with non-amended soil. Our findings further validate the potential role of OA and solarization in inducing soil suppressiveness, which contributes to sustainable management of soilborne pathogens.     

Effects of individual and combined use of bio-fumigation-derived products on the viability of Verticillium dahliae microsclerotia in soil

Verticillium dahliae is the causal agent of strawberry wilt. A microencapsulated terpene product containing cineole, camphor and borneol, digestate from anaerobic digestion, and BioFence™ derived from a mustard-based defatted seedmeal were tested for their suppressive activity against V. dahliae. First, naturally infested soil was amended with microencapsulated terpene, lavender waste pellet and BioFence™ (pellet) in a laboratory test to assess the efficacy against V. dahliae. Next, mini-field-plot experiments were conducted to evaluate the efficacy of individual and combined use of terpene, BioFence™ (liquid) and digestate against V. dahliae; sterile distilled water treatment and untreated control were also included. In the laboratory test, all treatments significantly reduced V. dahliae densities, with the control efficacy ranging from 27% (BioFence™) to 69% (lavender waste pellet). Although the lowest (1×) rate of terpene treatment resulted in a much lower control efficacy (35%) than the other two higher rates (3× – 55%; 9× – 53%), these differences were not statistically significant. In the field mini-plot trials, all treatments led to significant reductions in the V. dahliae density, with the efficacy ranging from 50% (digestate) to 78% (combined three-product treatment), irrespective of the initial wilt level. There were no significant differences in all comparisons of pairwise treatments except between digestate and combined three-product treatment. For the combined two or three-product treatments, the observed efficacy was significantly less than the expected efficacy on the assumption of Bliss independence. Furthermore, there were no significant differences between the observed efficacy of combined treatments and the best single component product efficacy. Although the observed efficacy for the combined three-product treatment was consistently higher than the best single component across replicate plots, such a difference was not statistically significant. The results indicate the value of these alternative treatments in practice but these are not likely to reduce V. dahliae inoculum sufficiently to eliminate the risk of strawberry wilt and question the value of combined treatments.     

The suppressive effect of compost amendments on Fusarium oxysporum f.sp. radicis-cucumerinum in cucumber and Verticillium dahliae in eggplant

The suppressive effect of six different compost amendments (A, B, C, D, E and Z) against Fusarium oxysporum f.sp. radicis-cucumerinum (Forc) in cucumber and Verticillium dahliae in eggplant was tested. The evaluation of composts in each pathosystem was carried out by recording external symptoms, isolations and plant growth. The concentration of total phenols was determined in the root and stem tissues of eggplant as well as in pure composts and in potting mixtures (soil amended with 20% of compost) at the beginning and the end of eggplant – V. dahliae bioassays. It was shown that composts A, B, C and D were effective against Forc, and composts C, D, E and Z were effective against V. dahliae. The decreased symptom severity and V. dahliae isolation ratio in eggplant was associated with significantly lower accumulation of phenols in stem tissues; whereas the concentration of total phenols in stem tissues of V. dahliae-infested eggplants was significantly higher compared to the non infested. In addition, total phenols content in the pure V. dahliae-suppressive composts was significantly higher than in the non-suppressive ones. Moreover, total phenols content in soil substrate and in potting mixture A decreased, whereas in mixtures B, C, D, E and Z was significantly increased during the time of bioassays process. Interestingly, total phenols content in the V. dahliae-suppressive potting mixtures C, D, E and Z was 3.8-, 3.7-, 3.7- and 4.4-fold higher compared with the non-suppressive control (100% soil), at the end of bioassays process (68 days post inoculation). This is the first insight in the role of phenols in the suppressive effect of composts against soil-borne pathogens, in planta.     

Synergistic interactions betweenRhizoctonia solani Kühn,Verticillium dahliae Kleb.,Meloidogyne spp. andPratylenchus neglectus (Rensch) Chitwood & Oteifa,in potato

Summary Samples of a sandy soil and a marine clay soil sterilized by steam were put in 55-1 containers insulated with polystyrene and placed outdoors on a brick pavement. Sandy soil was infested singly or in all possible combinations with root-knot nematodes (Meloidogyne spp.) and the fungiRhizoctonia solani andVerticillium dahliae, and the marine clay soil was infested with the root-lesion nematodePratylenchus neglectus and the same fungi to evaluate the effects of these organisms on the yield of potato. The experiments were carried out from 1983 to 1986. Tuber yield was reduced by single infestations of theMeloidogyne spp. andV. dahliae but not significantly byR. solani orP. neglectus. A three-factor interaction: nematode ×R. solani × V. dahliae was found in both experiments.R. solani andV. dahliae showed significant synergistic effects when soil was infested with theMeloidogyne spp. orP. neglectus.