Organic-matter effects on populations of dry rot of yam nematodes

The objective of this study was to evaluate the effect of organic-matter incorporated into the soil on population densities of the causal agents of the dry rot disease of yam, under field conditions. The experiment was performed in a natural infested area with a mixed population of Pratylenchus coffeae and Scutellonema bradys, in Quebrangulo county (Alagoas state, Brazil) in a randomized block design with five treatments and five replicates. The sources of organic matter used as soil amendments were: coconut husk powder, castor bean cake, cattle manure and chicken manure. Non amended soil was used as a control. Nine months after planting, the tubers were harvested. No statistical differences were found among disease incidence, yam production and nematode population densities in the soil. However, the application of chicken manure reduced P. coffeae population in tubers.

In 2014, African countries produced 65.7 million tons of tubers equivalent to 96.4% of the world production (68.2 million tons).Among South American countries, Brazil ranks second, with 25.5 thousand hectares and an estimated production of 247 thousand tons (FAO, 2015).According to Santos et al. (2011), the Northeastern region is the largest producer of yams in Brazil with approximately 15 thousand ha and production of 200 thousand tons (average yield of 10.5 t ha -1 ), mainly cultivated in the states of Paraíba, Pernambuco, Bahia and Alagoas.Among the constraints to yam production in Brazil, dry rot disease caused by Scutellonema bradys, Pratylenchus coffeae and Pratylenchus brachyurus causes the greatest damage to this crop (Moura, 2006).
In Alagoas, mixed populations are found in most yamgrowing areas with an incidence ranging between 0.2 and 85% (Muniz et al., 2012).
The first symptoms of the disease are light yellow lesions below the outer skin of the tubers, turning to a dark brown to black color as the disease progresses.External cracks arise in the tubers's skin and complete deterioration may occur during storage.The damage caused by the nematode is confined to sub-epidermal, peridermal and parenchymatous tissues extending to 1-2 cm into the tuber, although sometimes deeper (Kwoseh et al., 2002;Bridge and Starr, 2007).Above-ground symptoms are not apparent (Bridge and Starr, 2007).
The most successful method for preventing nematodes' damages remains in the use of nematode-free seed tubers in nematode-free land (Bridge and Starr, 2007), but the difficulty to obtaining healthy propagative material turn this technique unfeasible (Moura, 2006).
According Gowen et al. (2005) and McSorley ( 2011), an extensive range of organic materials have shown efficiency in reducing nematode populations in a number of pathosystems.Organic materials as agro-industrial and animal wastes may act as nutrient sources and improve water-holding capacity of the soil, increasing plant growth.Higher organic content in soils also stimulates the activity of plant-parasitic nematodes' antagonistic organisms.Furthermore, decomposition of residues results in the accumulation of specific compounds in the soils which may be nematicidal (Bridge, 1996).However, information on the use of organic materials in the management of plant-parasitic nematodes that affect yam crops is limited (Santos et al., 2009;Osei et al., 2013).
Thus, the aim of this work was to evaluate the effect of some animal and agro-industrial wastes that are available in the Northeast region of Brazil, to manage nematodes under field conditions.

MATERIALS AND METHODS
The experiment was conducted on February 2013, on a farm located in Quebrangulo county, AL (9° 15' 50.9"S; 36° 26' 11.7" W), previously grown with yam and naturally infested with dry rot disease nematodes.
The experiment was performed in a randomized block design with five treatments and five replicates, in plots consisting of four ridges 3.50 m long, considering the two central ridges as useful area.Seed tubers weighing approximately 250 to 350 g each were selected from a field known to be free of the dry rot disease (based on the absence of symptoms and after randomly sampling tubers to test presence of plant-parasitic nematodes), and planted at a 1.20 x 0.35 m spacing.
Previous to planting, a composite soil sample of four subsamples in each plot was collected, using a zig-zag pattern, in the two central rows to evaluate the initial nematode populations.The nematodes were extracted, according to the centrifugal-flotation technique (Jenkins, 1964), in aliquots of 100 cm 3 of soil and the nematodes were quantified with the aid of Peter's counting slides (Astel®, Botucatu-SP, Brazil), under a light microscope.The identification of Pratylenchus species was done according to Morais et al. 1495Gonzaga et al. (2012).In addition, the chemical and physical properties of soil were determined (Table 1), as well as the chemical analysis of the sources of organic matter (Table 2).Soil treatments included: 1) untreated soil (control), 2) coconut husk powder (from coconut epicarp) -37 t ha -1 , 3) castor bean cake -2.5 t ha -1 , 4) cattle manure -10 t ha -1 and 5) chicken manure -4 t ha -1 .These dosages were determined based on the results of the chemical analysis of soil and the nutrient content of the sources of organic materials, using a reference of 150 kg ha -1 nitrogen.Due to the high C/N ratio of coconut husk powder, a supplemental fertilizer with ammonium sulfate (10 g/plant) was applied to this treatment.Mineral fertilization with NPK 16-00-20 (20 g/plant) was made 70 days after planting, according to soil testing analysis.
Nine months after planting, the tubers were harvested and assessed for dry rot incidence (percentage of tubers exhibiting symptoms of the disease in relation to the total tubers evaluated) and tuber weight per plot, in a total of 18 plants.Mixed soil and tubers samples were taken to determine the final nematode populations, and were processed according to Jenkins (1964) and Coolen and D´Herde (1972), respectively.The nematode population densities were determined from duplicated 1 ml aliquots, in Peter's counting slide, under a light microscope, using a taxonomic key, as described early.The reproduction factor (RF) of the nematodes [RF = final population (tubers+soil)/initial population], in the different treatments was calculated according to Oostenbrink (1966).For statistical analyses, the data were transformed in log x or and means separated by Scott-Knott test at 5% probability, using the software Assistat 7.7 beta (Silva, 2014).

RESULTS AND DISCUSSION
The evaluation of the initial nematode populations showed the presence of P. coffeae and S. bradys with average of 12.0 to 25.0 specimens/100 cm 3 of soil among plots.There were no statistical differences, indicating the uniformity of the nematode populations in the field (Table 3).These low nematode population levels could be attributed to the stressful conditions due to the dry season.At the end of the experimental period, there was no significant difference among disease incidence, tuber production and nematode population densities in soil due to the use of different sources of organic matter.However, the application of chicken manure reduced P. coffeae population in tubers (Table 3).
Despite the efficiency of chicken manure in controlling nematode populations (Ferraz et al., 2010;Abdel-Dayem et al., 2012), negative results have also been observed.Examples can be found in Brazil, with Meloidogyne javanica and Meloidogyne incognita on bananas (Vilas Boas et al., 2004a;Vilas Boas et al., 2004b), and in the United States with Heterodera glycines in soybean crop (Donald et al., 2013).However, no published report on this source of organic matter was found regarding P. coffeae or S. bradys on yam plants.
In Nigeria, Adesiyan and Adeniji (1976) observed that application of cattle manure to the soil at a rate of 1.89 t ha -1 , increased tuber yield of D. alata, and significantly reduced the population density of S. bradys.The differences in the current results may possibly be related to the yam species and the implementation conditions on each experiment.Although, the mentioned work had been done under field conditions, the soil was artificially infested with the nematode.In addition, the authors reported that the average nematode population per 50 g of tuber peelings at harvest was 1,410 specimens which correspond to approximately 28 individuals per gram of tissue.Although, the application of the treatment significantly reduced the nematode population, according to Bridge et al. (2005), populations of S. bradys in excess of 20 nematodes/g of tuber peelings are necessary to produce external symptoms of damage.In the present work, up to 2,727 specimens of P. coffeae per gram of tuber peelings were recorded.
In Brazil, Santos et al. (2009) assessed the effect of antagonistic plants used as green manure and organic wastes to control nematodes in yam (D. cayenensis), under field conditions.The authors observed incidence of dry rot disease of 36.35% in the first year of cropping and 21.88% in the second, with the use of cattle manure.However, data on the initial nematode populations in the area, the rate of manure application, and the disease incidence from the control plants, which prevent comparisons between results were not shown.
With respect to by-products from the processing of coconut fruits, the data on its use in the management of plant-parasitic nematodes are scarce, and when assessed for controlling M. javanica in banana (Vilas Boas et al., 2004a) and tomato crops (Dallemole-Giaretta et al., 2010), only in the second case was observed a favorable result.Silveira et al. (2002) detected high quantity of microorganisms in coconut coir fiber (from coconut mesocarp) reported as agents of biocontrol for several pathogens, among these are Trichoderma species.According to Meyer et al. (2000), culture filtrate from T. virens contained extracellular factors that inhibited egg hatch and second-stage juvenile mobility of M. incognita.
Concerning the use of castor bean cake to control plant-parasitic nematodes, some researches have been already published.For example, this organic material has been applied to the soil for management of Meloidogyne species in sugarcane (Dinardo-Miranda and Fracasso, 2010), and tomato crops (Lopes et al., 2009;Roldi et al., 2013).According to Rich et al. (1989) the nematicidal activity of this product was attributable to the chemical compound, ricin, a natural occurring lectin capable of inhibiting protein synthesis (Audi et al., 2005).The results obtained in the present work are not in accordance with this.
The difference between the reports shown in the literature and the current work could be due to variations in nematode species, host plant, chemical composition of the organic amendments as well as the rate and time of Table 3.Initial population of the causal agents of dry rot disease in 100 cm 3 of soil in naturally infested field where different organic materials were incorporated to the soil; dry rot incidence; yam production nine months after planting and number of nematodes in 100 cm 3 of soil and 20 g of tubers.Quebrangulo, AL, 2013.Averages of five replicates; data followed by the same letter within a column do not differ at 5% probability level by Scott-Knott test.a , b analyses of variance with the data converted into log x and , respectively.Mean-square residue (MSR); *significant at 5% probability by F test; ns not significant to probability higher than 5% by F test.CV = coefficient of variation.application, and the environmental factors like temperature, microbial community and soil type.In addition, the present work involved a mixed nematode population.

Conclusion
The use of organic materials as cattle manure, chicken manure, castor bean cake and coconut husk powder did not reduce the incidence of dry rot of yams under field conditions.However, the application of chicken manure reduced P. coffeae population in tubers.

Table 1 .
Chemical and physical properties of soil collected in Quebrangulo county, AL, naturally infested with a mixed population of Pratylenchus coffeae and Scutellonema bradys.

Table 2 .
Chemical characteristics of the organic materials used in the experiment.