First report of the maize cyst nematode Heterodera zeae in Sichuan Province of Southwest China

Maize is the largest crop planted in China. Nine species of cyst nematodes have been reported to affect maize production. Heterodera zeae, H. avenae and Punctodera chalcoensis can cause significant maize yield losses annually (Luc et al. 2005). In 1971, the maize cyst nematode H. zeae was first detected in Rajasthan, India (Koshy et al. 1971). Subsequently, it has been reported in many other countries such as the United States, Greece, Pakistan, and Egypt. In China, H. zeae was first identified in the maize fields of Laibin City, Guangxi Zhuang Autonomous Region (Wu et al., 2017). Cui et al. (2020) identified H. zeae in a maize field of Yuzhou City, Henan Province of Central China in 2018. From 2018 to 2022, a survey of cyst-forming nematodes was conducted in Southwest China. Fifteen soil samples of about 500 g each were collected from Luding County, Ganzi Prefecture of Sichuan Province. No major aboveground symptoms were shown on maize, but a few females were observed on the roots of maize in one field. The cysts and second-stage juveniles (J2s) were collected from each soil sample using Cobb's screening gravity method. A total of 8.50±2.0 cysts per 100 ml of soil on the average were observed in the field. A thin subcrystalline layer was discernible only in young cysts. Morphological and molecular studies of cysts and J2s indicated that the nematodes were identified to be H. zeae in a maize-field. Morphologically, the cysts were in a lemon shape, light brown or pearly white in color. The vulval cone was prominent. Fenestra ambifenestrate, and semifenestra were separated by a fairly wide vulval bridge, fenestral length and width were variable, and the cyst wall was shown in a zigzag pattern. The J2s' body was in a vermiform, tapering at both ends, with a hyaline tail. Stylet was strongly developed with round or slightly anteriorly directed knobs. Morphological measurements of the cysts (n = 9) determined that the mean body length was 417.2 μm (403.6 to 439.4 μm), body width was 429.7 μm (397.6 to 456.9μm); length-width ratio was 1.4 (0.75 to 3); fenestra length was 525.3 μm (498.5 to 570.7 μm); and the mean semifenestra width was 458.6 μm (403.6 to 546.3 μm). Morphometric measurements of second-stage juveniles (n = 20) showed a body length of 419.7μm (355.8 to 492.5 μm); a stylet length of 20.8 μm (19.51 to 23.3μm); a tail length of 41.5 μm (20 to 49.4 μm); and a hyaline tail length of 20.7 μm (16.6 to 24 μm). The main morphological characteristics and measured values were basically consistent with those described by Cui et al. (2022), and all of which were similar to those of H. zeae. Amplification of DNA from random single cysts (n = 5) was conducted using the protocol described by Cui et al. (2022). The rDNA-internal transcribed spacer (ITS) was amplified and sequenced using a pair of universal primers TW81 (5'-GTTTCCGTAGGTGAA CCTGC-3') and AB28 (5'-ATATGCTTAAGTTCAGCGGGT-3'). The ITS sequences were deposited at GenBank with the accession number OR811029.1. Alignments of sequences showed an identity of 98% with H. zeae sequences from China (OP692769.2, MW785772.1) and the USA (GU145616.1), which were confirmed using a pair of species-specific primers HzF1 (5'-GGGGAGGTGAATGTGGG-3') and HzR1 (5'-CCTTTGGCAATCGGTGA-3') of H. zeae with a targeted PCR fragment of 393 bp (Cui et al. 2022). Pathogenicity was conducted and confirmed by infection and reproduction on maize. Seeds (cv. Zhengda 619) were sown in three pots that contained 150 ml of a sterile soil mixture (loamy soil: sand=1:1), and 5 cysts (103 eggs/cyst on the average) were inoculated in each pot at 25/30°C, under a 12-h dark/12-h light condition (Cui et al. 2023). Fifteen days after sowing, third- and fourth-stage juveniles were observed in the rootstained with acid fuchsin, and a total of 32 cysts per maize plant on the average were collected at 40 days after sowing. The new cysts' morphological and molecular characteristics were identical to the cysts from the original soil samples. To the best of our knowledge, this is the first report of H. zeae as a pathogen on maize in Sichuan Province, Southwest China. Our findings will be useful for management and further research of maize cyst nematodes.

Characterization of Heterodera zeae Populations from Portugal

Three populations of the corn cyst nematode Heterodera zeae, one found in the rhizosphere of a fig tree and two infecting corn, were studied using the morphology and morphometry of cysts and second-stage juveniles, and compared with other populations. The intrapopulation and intraspecific variability are discussed. A simple and improved technique to prepare vulval cones for SEM is described. The non-specific esterase patterns of females, isolated from infected corn, were analyzed by electrophoresis in polyacrylamide gels. Two bands of esterase activity were detected. The occurrence of H. zeae is reported for the first time in Portugal and Europe.

Survey of crop losses in response to phytoparasitic nematodes in the United States for 1994

Previous reports of crop losses to plant-parasitic nematodes have relied on published results of survey data based on certain commodities, including tobacco, peanuts, cotton, and soybean. Reports on crop-loss assessment by land-grant universities and many commodity groups generally are no longer available, with the exception of the University of Georgia, the Beltwide Cotton Conference, and selected groups concerned with soybean. The Society of Nematologists Extension Committee contacted extension personnel in 49 U.S. states for information on estimated crop losses caused by plant-parasitic nematodes in major crops for the year 1994. Included in this paper are survey results from 35 states on various crops including corn, cotton, soybean, peanut, wheat, rice, sugarcane, sorghum, tobacco, numerous vegetable crops, fruit and nut crops, and golf greens. The data are reported systematically by state and include the estimated loss, hectarage of production, source of information, nematode species or taxon when available, and crop value. The major genera of phytoparasitic nematodes reported to cause crop losses were Heterodera, Hoplolaimus, Meloidogyne, Pratylenchus, Rotylenchulus, and Xiphinema.

Identification of Cyst Nematodes of Agronomic and Regulatory Concern with PCR-RFLP of ITS1

The first internally transcribed spacer region (ITS1) from cyst nematode species (Heteroderidae) was compared by nucleotide sequencing and PCR-RFLP. European, Asian, and North American isolates of five heterodefid species were examined to assess intraspecific variation. PCR-RFLP patterns of amplified ITS1 DNA from pea cyst nematode, Heterodera goettingiana, from Northern Ireland were identical with patterns from Washington State. Sequencing demonstrated that ITS1 heterogeneity existed within individuals and between isolates, but did not result in different restriction patterns. Three Indian and two U.S. isolates of the corn cyst nematode, Heterodera zeae, were compared. Sequencing detected variation among ITS1 clones from the same individual, between individuals, and between isolates. PCR-RFLP detected several restriction site differences between Indian and U.S. isolates. The basis for the restriction site differences between isolates from India and the U.S. appeared to be the result of additional, variant ITS1 regions amplified from the U.S. isolates, which were not found in the three India isolates. PCR-RFLP from individuals of the U.S. isolates created a composite pattern derived from several ITS1 types. A second primer set was specifically designed to permit discrimination between soybean (H. glycines) and sugar beet (H. schachtii) cyst nematodes. Fok I digestion of amplified product from soybean cyst nematode isolates displayed a uniform pattern, readily discernible from the pattern of sugar beet and clover cyst nematode (H. trifolii).

Factors Influencing Emergence of Juveniles from Cysts of Heterodera zeae

Several factors were studied to determine their effects on hatch and emergence of second-stage juveniles (J2) from cysts of Heterodera zeae. The optimum temperature for emergence of J2 from cysts of H. zeae was 30 C. No juveniles emerged from cysts at 10 or 40 C. Immersion of cysts in 4 mM zinc chloride solution stimulated 10% greater emergence of J2 than occurred in tap water controls during 28 days. Fresh corn rhizosphere leachates from 25-day and older plants growing in sand or sandy field soil stimulated 22-24% greater emergence of J2 from cysts than occurred in tap water after 28 days. Rhizosphere leachates stored for 30 days at 4 C and leachates of sand, sandy field soil, and silty field soil inhibited emergence of J2 from cysts by 7-12% compared to tap water. Rhizosphere leachates from corn plants aged 20, 30, 40, 50, or 60 days growing in sandy field soil stimulated emergence of J2 from cysts. Similar numbers of J2 emerged from cysts regardless of whether the source of cysts was field microplot cultures, greenhouse cultures, or growth chamber cultures. Fertilizing growth chamber cultures of H. zeae on corn plants resulted in a doubling of the numbers of cysts produced in the cultures, and those cysts yielded 2-3 times as many emerged J2 in hatching tests compared to cysts from similar unfertilized cultures.

Resistance in Zea mays to Heterodera zeae

Twenty-three precommercial field corn lines (Zea mays) were screened in the greenhouse and in vitro for the ability to support reproduction of Heterodera zeae. Although H. zeae reproduced on all corn lines, reproduction was only 0.4 to 4.5% on the five least suitable corn lines in greenhouse tests compared with the susceptible check line Pioneer brand 3184. The least suitable experimental line supported an average of 30 cysts plus females after 8 weeks growth, whereas the susceptible check, Pioneer brand 3184, averaged 8,183 cysts plus females per pot. Reproduction of H. zeae in in vitro root cultures of the 23 lines and susceptible check cultivar, Iochief, was too low to be of any value in detecting resistance to this nematode under the conditions of these tests.

Temperature and the Life Cycle of Heterodera zeae

Development of the corn cyst nematode, Heterodera zeae, was studied in growth chambers at 20, 25, 29, 33, and 36 +/- 1 C on Zea mays cv. Pioneer 3184. The optimum temperature for reproduction appeared to be 33 C, at which the life cycle, from second-stage juvenile (J2) to J2, was completed in 15-18 days; at 36 C, 19-20 days were required. Juveniles emerged from eggs within 28 days at 29 C and after 42 days at 25 C. Although J2 were present within eggs after 63 days at 20 C, emergence was not observed up to 99 days after inoculation. Female nematodes produced fewer eggs at 20 C than at higher temperatures.

Survival of Heterodera zeae in Soil in the Field and in the Laboratory

Eggs and (or) second-stage juveniles (J2) inside cysts of Heterodera zeae survived over winter in the field with no detectable mortality at all six depths to 30 cm from which soil samples were collected between corn stubble in the row at 4-8-week intervals. Few or no free J2 were recovered from soil collected in January-April from the top 5 cm, but some were recovered at all samplings from soil collected at greater depths. Emergence of J2 from cysts and numbers of females developing on corn roots in bioassays of cysts increased substantially between January and April. Cyst numbers in a fallow area of the corn field did not decline at any depth to 30 cm during 20 months. Free soil J2, J2 emerged from cysts, and females from the bioassay of cysts were highest at the first soil sampling in July after 10 months of fallow; numbers of nematodes in all three categories declined thereafter, but a few were still detectable after 20 months of fallow. Some cysts were still being recovered after 51 months from naturally infested field soil stored moist in the laboratory at 2 C and 24 C. Females were produced in the bioassays of cysts recovered from soil stored for 38 months at 24 C and for 32 months at 2 C. No free J2 were recovered from soil after 1 month of storage at -18 C, but even after 7 months storage J2 emerged from cysts that were recovered and many females developed in bioassays of those cysts.

Investigations of the Host Range of the Corn Cyst Nematode, Heterodera zeae, from Maryland

The host range of the corn cyst nematode, Heterodera zeae, recently detected in Maryland, was investigated. A total of 269 plant entries, representing 68 families, 172 genera, and 204 species, was inoculated with cysts or a mixture of eggs and second-stage juveniles of H. zeae. The host range of the Maryland population of H. zeae was limited to plants of the Gramineae and included 11 tribes, 33 genera, 42 species, and 77 entries. All 22 corn (Zea mays) cultivars tested were hosts. Other economic hosts included certain cultivars of barley (Hordeum vulgare), oat (Arena sativa), rice (Oryza sativa), sorghum (Sorghum bicolor), sugar cane (Saccharum interspecific hybrid), and wheat (Triticum aestivum). Fall panicum (Panicum dichotomiflorum), a weed species common to cultivated fields in Maryland, was also a host for H. zeae. Other hosts included meadow foxtail (Alopecurus pratensis), Calamagrostis eipgeios, Job's tears (Coix Lachryma-Jobi), green sprangletop (Leptochloa dubia), witchgrass (Panicum capillare), broomcorn (Panicum miliaceum), fountain grass (Pennisetum rueppeli), reed canary grass (Phalaris arundinacea), common reed (Phragmites australis), eastern gamagrass (Tripsacum dactyloides), corn (Zea mays), and teosinte (Zea mexicana).