Pathological Differences in Heterodera schachtii Populations

Five populations of Heterodera schachtii Schm. from Oregon, Idaho, and Utah did not differ significantly in seedling penetration and rate of emergence and virulence. Another Utah H. schachtii population (Utah 2), however, differed from these five populations in all of the above-mentioned characteristics. More H. schachtii larvae of the Utah 2 population than the other populations penetrated sugarbeet seedlings at 10, 15, 20, and 25 C. Root and top weights of sugarbeet plants were signiticantly less when roots were parasitized by the Utah 2 population than when they were parasitized by larvae of the other nematode populations under similar experimental conditions. Also, the period of larval emergence was shorter in the Utah 2 population than in any of the other H. schachtii populations.     

Differences in the Response of Certain Weed Host Populations to Heterodera schachtii

Significant differences (P = 0.05) in nematode reproduction were observed among populations of Heterodera schachtii and weed collections of black nightshade, common lambsquarters, common purslane, redroot-pigweed, shepherdspurse, and wild mustard from Colorado, Idaho, Oregon, and Utah. Colorado weeds supported the greatest nematode development (P = 0.05). Weeds collected from Idaho and Utah were similar with respect to their response to H. schachtii with the exception of shepherdspurse. At increasing soil temperatures, a Utah redroot-pigweed collection showed a higher percent susceptibility to a Utah nematode population than to nematode populations from the other states (P = 0.05). There was a higher percentage of susceptible plants when the weed host population was collected from the same geographical area as the nematode inoculun.     

Host-parasite Interactions of Pratylenchus scribneri on Selected Crop Plants

Greenhouse tests were conducted to determine the effects of soil temperature and texture on development of Pratylenchus scribneri and the pathogenicity and reproductive rates of this nematode on selected crop plants. In a sandy loam soil, greatest numbers of P. scribneri were found at 30 and 35 C on sudangrass and sugarbeet, respectively. In a silty clay loam, the nematode reproduced best at 35 C on sugarbeet. Higher populations of P. scribneri were found in the sandy loam than silty clay loam soil at corresponding temperatures. In a pathogenicity test, top and root growth of sudangrass and barley were suppressed by the nematode, whereas no significant growth inhibition was found on wheat and alfalfa. Tests with other vegetable and field crops indicated wide variance in nematode reproduction.     

Parasitism of Nonhost Cultivars by Ditylenchus dipsaci

The alfalfa race of Ditylenchus dipsaci parasitized and caused characteristic symptoms on nonhost seedlings of sweet clover, onion, tomato, sugarbeet, and wheat in controlled growth-chamber studies. Although the nematode was unable to reproduce on any of the cultivars, it caused plant mortality ranging from 20% on sugarbeet and tomato to 100% on onion.     

Tolerance Limit of the Sugarbeet to Heterodera schachtii

Field and greenhouse experiments showed that yield losses of sugarbeet, Beta vulgaris, did not occur in soil infested with fewer than eight Heterodera schachtii eggs/g soil. However, larger population densities greatly reduced sugarbeet yield. In the field experiment, the yield in microplots inoculated with more than 64 eggs/g soil was less than 20% of yields in uninoculated microplots. Nevertheless, tolerance limits of 4 and 1.8 eggs/g soil, in greenhouse and field microplots, respectively, were derived by fitting the data with the equation y =m + (l - m)z^P-T. Maximum rates of multiplication of 55 and more than 300, and equilibrium densities of 340 and 130 eggs/g soil, were estimated in greenhouse and field microplot tests, respectively.     

Influence of Selected Cultural Practices on Winter Survival of Pratylenchus brachyurus and Subsequent Effects on Soybean Yield

Planting date of soybean, Glycine max, influenced winter survival of Pratylenchus brachyurus in microplots at two locations in North Carolina. Delayed planting resulted in a linear decrease (P = 0.05) in the numbers of P. brachyurus at soybean harvest. Effects of planting date on nematode numbers persisted over winter, indicating that survival in the absence of a host is density independent. Compared with winter fallow, winter wheat, Triticum aestivum, reduced winter survival of P. brachyurus. Subsequent soybean yields were suppressed by the overwintering population of this nematode at one location but not at another.     

Suppression of Root-knot Nematode Populations with Selected Rapeseed Cultivars as Green Manure

Meloidogyne chitwoodi races 1 and 2 and M. hapla reproduced on 12 cultivars of Brassica napus and two cultivars of B. campestris. The mean reproductive factors (Rf), Rf = Pf at 55 days ÷ 5,000, for the three nematodes were 8.3, 2.2, and 14.3, respectively. All three nematodes reproduced more efficiently (P < 0.05) on B. campestris than on B. napus. Amending M. chitwoodi-infested soil in plastic bags with chopped shoots of Jupiter rapeseed reduced the nematode population more (P < 0.05) than amendment with wheat shoots. Incorporating Jupiter shoots to soil heavily infested with M. chitwoodi in microplots reduced the nematode population more (P < 0.05) than fallow or corn shoot treatments. The greatest reduction in nematode population density was attained by cropping rapeseed for 2 months and incorporating it into the soil as a green manure.     

The Interrelationship of Heterodera schachtii and Ditylenchus dipsaci on Sugarbeet

Heterodera schachtii significantly (P = 0.05) reduced sugarbeet root growth below that of uninoculated controls at 20, 24, and 28 C, and Ditylenchus dipsaci significantly (P = 0.05) reduced root growth below that of uninoculated controls at 16, 20, 24, and 28 C. A combination of H. schachtii and D. dipsaci significantly (P = 0.05) reduced root growth below that of single inoculations of H. schachtii at all temperatures and D. dipsaci at 20, 24, and 28 C. Single inoculations of H. schachtii and D. dipsaci significantly (P = 0.05) reduced top growth of sugarbeet below that of uninoculated controls at 20, 24, and 28 C, and 16, 20, 24, and 28 C, respectively. A combination of the two nematodes significantly (P = 0.05) reduced top growth below that of single inoculations of H. schachtii at all temperatures. However, a combination of the two nematodes failed to significantly (P = 0.05) reduce top growth below that of single inoculations of D. dipsaci at any temperature. Inoculations of either H. schachtii or D. dipsaci did not affect penetration of the other nematode, and D. dipsaci did not affect development and reproduction of H. schachtii. D. dipsaci did not reproduce on sugarbeet.     

Effects of Soil Moisture on Control of Heterodera schachtii with Aldicarb

Soil moisture and the nematode population density in aldicarb-treated soil influenced control of the sugarbeet nematode, Heterodera schachtii. Greater numbers of nematode larvae infected 14-day-old sugarbeet seedlings growing in aldicarb-treated soil at 20-30% than at 80-100% field capacity (F. C.), and plant growth was inversely related to nematode infection and the nematode population density. Compared with that of control plants, plant growth increase also was greater at 80-100% F. C. when the nematode population was above 1.8 larvae/gm soil. A nematode population of 1.8 larvae/gm soil did not significantly affect sugarbeet yields. Aldicarb gave less control when soil moisture levels dropped to 20 and 50% F. C. at nematode populations of 3.5 and 6.2 larvae/gm soil. More effective control was obtained wth soil moisture levels at or above 80% F. C. This difference was attributed to continued activity of the toxicant in the rhizosphere at the high moisture level.     

The Relationship of Plant Age,Soil Temperature,and Population Density of Heterodera schachtii on the Growth of Sugarbeet

There were direct relationships between inoculum density of Heterodera schachtii Schm. (nematode population density), initial soil temperature, the growth of sugarbeets in the greenhouse under controlled temperatures, and nematode populations. Heterodera schachtii was least pathogenic on plants inoculated at 6 wk of age and most pathogenic on plants grown from inoculated germinated seed (0 wk of age). In the field, H. schachtii was least pathogenic on sugarbeets grown at an initial soil temperature of 6 C and most pathogenic on those grown at an initial soil temperature of 24 C. The growth period for sugarbeets at the different soil temperatures was determined by heat units; since penetration of sugarbeet roots by H. schachtii larvae is accelerated at soil temperatures above 10 C, each hour-degree ahove 10 C was counted as one effective heat unit (HU). Using this guideline it was determined that root weight depressions in the greenhouse, for each degree-unit population (HU-UP) where unit population = one larvae/g soil, were 0.052, 0.09, 0.12, and 0.17 mg at initial soil temperatures of 6, 12, 18, and 24 C, respectively. Root weight depressions were 0.28, 0.23, 0.15, and 0.086 mg when plants were inoculated at 0, 2, 4, and 6 wk of age.     

Sequential Decision Rules for Managing Nematodes with Crop Rotations

A dynamic model of nematode populations under a crop rotation that includes both host and nonhost crops is developed and used to conceptualize the problem of economic control. The steady state of the dynamic system is used to devise an approximately optimal decision policy, which is then applied to cyst nematode (Heterodera schachtii) control in a rotation of sugarbeet with nonhost crops. Long-run economic returns from this approximately optimal decision rule are compared with results from solution of the exact dynamic optimization model. The simple decision rule based on the steady state provides long-run average returns that are similar to the fully optimal solution. For sugarbeet and H. schachtii, the simplified rule can be calculated by maximizing a relatively simple algebraic expression with respect to the number of years in the sequence of nonhost crops. Maximization is easy because only integers are of interest and the number of years in nonhost crops is typically small. Solution of this problem indirectly yields an approximation to the optimal dynamic economic threshold density of nematodes in the soil. The decision rule requires knowledge of annual nematode population change under host and nonhost crops, and the relationship between crop yield and nematode population density.     

Reproduction of Heterodera schachtii Schmidt on Resistant Mustard,Radish, and Sugar Beet Cultivars

The reproduction of a Wyoming population of Heterodera schachtii was determined for resistant trap crop radish (Raphanus sativus) and mustard (Sinapis alba) cultivars, and resistant and susceptible sugar beet (Beta vulgaris) cultivars in a greenhouse (21 °C/16 °C) and a growth chamber study (25 °C). Oil radish cultivars also were field tested in 2000 and 2001. In the greenhouse study, reproduction was suppressed similarly by the resistant sugar beet cultivar Nematop and all trap crop cultivars (P ≤ 0.05). In the growth chamber study, the radish cultivars were superior to most of the mustard cultivars in reducing nematode populations. All trap crops showed less reproduction than Nematop (P ≤ 0.05). In both studies, Nematop and all trap crops had lower Pf than susceptible sugar beet cultivars HH50 and HM9155 (P ≤ 0.05). In field studies, Rf values of radish cultivars decreased with increasing Pi of H. schachtii (r² = 0.59 in 2000 and r² = 0.26 in 2001). In 2000, trap crop radish cv. Colonel (Rf = 0.89) reduced nematode populations more than cv. Adagio (Rf = 4.67) and cv. Rimbo (Rf = 13.23) (P ≤ 0.05) when Pi was lower than 2.5 H. schachtii eggs and J2/cm³ soil. There were no differences in reproductive factors for radish cultivars in 2001 (P ≤ 0.05); Rf ranged from 0.23 for Adagio to 1.31 for Commodore for all Pi.     

Effects of Cropping Sequences on Population Densities of Meloidogyne hapla and Carrot Yield in Organic Soil

The influence of various cropping sequences on population densities of Meloidogyne hapla and carrot yield was studied in organic soil under microplot-and field conditions. Spinach, radish, barley, oat, and wheat were poor or nonhosts for M. hapla. Population densities of M. hapla were maintained or increased on cabbage, celery, lettuce, leek, marigold, and potato. Marketable percent-age and root weight of carrots were greater following spinach, oat, radish, and fallow-onion than those following two crops of onion or carrot in microplots. Under field conditions, the carrot-onion-oat-carrot cropping sequence decreased M. hapla population densities and provided a 282% increase in marketable yield of carrot compared to a carrot monoculture. Two consecutive years of onion increased M. hapla population densities causing severe root galling and a 50% yield loss in the following crop of carrot. Based on root-gall indices, carrots could be grown economically for 2 years following radish, spinach, and oat, but not following onion and carrot without the use of nematicides.     

Factors Affecting the Biology and Pathogenicity of Heterodera schachtii on Sugarbeet

A direct relationship exists between soil temperature and Heterodera schachtii development. The average developmental period of two nematode populations from Lewiston, Utah, and Rupert, Idaho, from J2 to J3, J4, adult, and the next generation J2 at soil temperatures of 18-28 C were 100, 140,225, and 399 degree-days (base 8 C), respectively. There was a positive relationship (P < 0.05) between nematode Pi, nematode generations, and sugarbeet yields. The greatest sugarbeet growth inhibition (87%) occurred when sugarbeets were exposed to a Pi of 12 eggs/cm³ soil for five generations (1,995 degree-days), compared with a 47% inhibition when plants were exposed to the same Pi for two generations. There was a negative correlation (P < 0.05) between the Pi, Pf, and sugarbeet yield for each population threshold. The smaller the Pi, the greater the sugarbeet yields and the greater the Pf. Root yields were 80 and 29 t /ha and Pf were 8.4 and 3.6 eggs/cm³ soil when sugarbeet seeds were planted at Pi of 0.4 and 7.9 eggs/cm³. respectively, at a soil temperature of 8 C. The number of years rotation with a nonhost crop required to reduce the nematode population density below a damage threshold level of 2 eggs/cm³ depends on the Pi. A Pi of 33.8 eggs/cm³ soil required a 5-year crop rotation, whereas a Pi of 8.4 eggs/cm³ soil required a 2-year crop rotation.     

Penetration,Development, and Reproduction of Heterodera schachtii on Fagopyrum esculentum,Phacelia tanacetifolia,Raphanus sativus,Sinapis alba,and Brassica oleracea

The penetration, development, and reproduction of a California population of the sugarbeet cyst nematode, Heterodera schachtii, was observed on cultivars of cabbage (Brassica oleracea), phacelia (Phacelia tanacetifolia), buckwheat (Fagopyrum esculentum), oilseed radish (Raphanus sativus), and white mustard (Sinapis alba). With the exception of the nonhost, phacelia, all were readily penetrated by second-stage juveniles of H. schachtii. After 38 days at 25 C, no cysts were observed on phacelia cv. Angelia or on the oilseed radish cv. Nemex and Pegletta. Cyst production was low (<2.5 cysts/plant) on the buckwheat cv. Tardo and Prego and most of the oilseed radish cultivars. Cyst production was intermediate (5-14 cysts/plant) on most of the white mustard cultivars, and high on cabbage (20-110 cysts/plant). In microplot studies conducted over 133 days (approx. 450 degree-days, base 8 C), the reproductive index for H. schachtii was greater than 1.0 for cultivars of phacelia, oilseed radish, and white mustard as welt as in fallow treatments, indicating the need for further research on the use of these crops under field conditions.     

A Simulation Model of Heterodera schachtii Infecting Beta vulgaris

A simulation model of a single sugarbeet, Beta vulgaris L., plant infected by the sugarbeet cyst nematode, Heterodera schachtii Schmidt, was developed using published information. The model is an interactive computer simulation programmed in FORTRAN. Given initial population densities of the nematode at planting, the model simulates nematode population dynamics and the growth of plant tap and fibrous roots. The driving variable for nematode development and plant growth is temperature.     

Interaction of Ditylenchus dipsaci and Meloidogyne hapla on Resistant and Susceptible Plant Species

Numbers ofDitylenchus dipsaci or Meloidogyne hapla invading Ranger alfalfa, Tender crop bean, Stone Improved tomato, AH-14 sugarbeet, Yellow sweet clover, and Wasatch wheat from single inoculations were not significantly different from numbers by invasion of combined inoculations. D. dipsaci was recovered only from shoot and M. hapla only from root tissue. Combined inoculations did not affect reproduction of either D. dipsaci or M. hapla. D. dipsaci suppressed shoot growth of all species at 15-30 C, and M. hapla suppressed shoot growth of tomato, sugarbeet, and sweet clover at 20, 25, and 30 C. There was a positive correlation (P < 0.05) between shoot and root growth suppression by D. dipsaci on all cultivars except wheat at 20 C and tomato at 30 C. M. hapla suppressed (P < 0.05) root growth of sugarbeet at 20-50 C and wheat at 30 C. Growth suppression was synergistic in combined inoculations of sweet clover shoot growth at 15 C and root growth at 20-30 C, wheat root growth at 15 and 20 C, and tomato root growth at 15-30 C (P < 0.05) D. dipsaci invasions caused mortality of alfalfa and sweet clover at 15-30 C and sugarbeet at 20-30 C. Mortality rates of alfalfa and sweet clover increased synergistically (P < 0.05) from combined inoculations.     

Importance of Temperature in the Pathology of Meloidogyne hapla and M. chitwoodi on Legumes

Effects of temperatures on the host-parasite relationships were studied for three legume species and four populations of root-knot nematodes from the western United States. The nematode populations were Meloidogyne hapla from California (MHCA), Utah (MHUT), and Wyoming (MHWY), and a population of M. chitwoodi from Utah (MCUT). The legumes were milkvetch (Astragalus cicer), alfalfa (Medicago sativa), and yellow sweet clover (Melilotus officinalis). All milkvetch plants survived inoculation with all nematode populations, while alfalfa and yellow sweet clover were more susceptible. On yellow sweet clover, MHCA was most pathogenic at 30 °C based on suppression of shoot growth while MHUT, MHWY, and MCUT were most pathogenic at 25 °C. All nematode populations suppressed growth of yellow sweet clover more than growth of milkvetch and alfalfa. The reproductive factor (Rf = final nematode population/initial nematode population) of MHCA was positively correlated (r = 0.83) with temperature between 15 °C and 30 °C. The greatest Rf occurred on alfalfa inoculated with MHCA at 30 °C. The Rf of MHUT, MHWY, and MCUT were positively correlated (r= 0.76, r= 0.78, and r= 0.73, respectively) with temperature between 15 °C and 25 °C. The Rf values of MHUT and MHWY were similar on all species and exceeded the Rf of MCUT at all temperatures (P < 0.05).     

Pathological Interaction of a Combination of Heterodera schachtii and Meloidogyne hapla on Tomato

Increased culturing of a tomato population of Heterodera schachtii (UT1C) on tomato for 480 days (eight inoculation periods of 60 days each) significantly increased virulence to ''Stone Improved'' tomato. A synergistic relationship existed between Meloidogyne hapla and H. schaehtii on tomato. A combination of H. schachtii (UTIC) and M. hapla significantly reduced tomato root weights by 65, 64, and 61% below root weights of untreated controls, and single inoculations of M. hapla and H. schachtii, respectively. This corresponded to root reductions of 42, 44, and 46% from a combination of H. schachtii (UT1B) and M. hapla. Antagonism existed between H. schachtii and M. hapla with regard to infection courts and feeding sites. The root-knot galling index dropped from 6.0 with a single inoculation of M. hapla to 4.3 and 3.3 with combined inoculations of M. hapla plus UT1B and M. hapla plus UTIC cyst nematode populations. The pathological virulence of H. schachtii to sugarbeet was not lost by extended culturing on tomato; there were no differences in penetration, maturation, and reproduction between sugarbeet populations continually cultured on sugarbeet and the population continually cultured on tomato.     

Parasitism of Heterodera schachtii and Meloidogyne javanica by Hirsutella rhossiliensis in Microplots over Two Growing Seasons

Numbers of cyst and root-knot nematodes and percentage parasitism by the nematophagous fungus Hirsutella rhossiliensis were quantified in microplots over 2 years. The microplots contained either sugarbeets in loam infested with Heterodera schachtii or tomatoes in sand infested with Meloidogyne javanica. The fungus was added to half of the microplots for each crop. Although H. rhossiliensis established in both microplot soils, the percentage of nematodes parasitized did not increase with nematode density and nematode numbers were not affected by the fungus. The results indicate that long-term interactions between populations of the fungus and cyst or root-knot nematodes will not result in biological control.