Cereal cyst nematodes (CCNs) can cause significant economic yield losses alone or in combination with other biotic and abiotic factors. The damage caused by these nematodes can be enormous when they occur in a disease complex, particularly in areas subject to water stress. Of the 12 valid CCN species, Heterodera avenae, H. filipjevi, and H. latipons are considered the most economically important in different parts of the world. This paper reviews current approaches to managing CCNs via genetic resistance, biological agents, cultural practices, and chemical strategies. Recent research within the soil borne pathogen program of the International Maize and Wheat Improvement Center has focused on germplasm screening, the potential of this germplasm as sources of resistance, and how to incorporate new sources of resistance into breeding programs. Breeding for resistance is particularly complicated and difficult when different species and pathotypes coexist in nature. A lack of expertise and recognition of CCNs as a factor limiting wheat production potential, combined with inappropriate breeding strategies and slow screening processes limit genetic gains for resistance to CCNs.

Tbe distribution of important plant-parasitic and free-living nematodes in tbe cereal production areas of tbe Central Anatolian Plateau (CAP) of Turkey was investigated witb systematic surveys. Two important plant-parasitic nematode groups were found widely distributed; cereal-cyst nematodes (78.3%) and root-lesion nematodes (42.6%). Cereal cyst nematodes (CCN) were identified as Heterodera filipjevi in 18 provinces. Heterodera latipons was found in only one province. Pratylenchus thornei and P. neglectus were tbe most widely distributed species of root-lesion nematodes. Otber frequently recorded plant-parasitic nematodes belonged to tbe genera Geocenamus (52.4%), Pratylenchoides (35.6%), Helicotyienchus (29.7%) and Paratylenchus (19.2%). Konya on tbe soutbern CAP bad a significantly bigb incidence of P. neglectus as well as free-living nematodes. Tbe incidence of CCN was greatest in areas of sandy soils on tbe CAP, witb densities of up to 95 cysts (100 g soil)"'. Population densities of Geocenamus, Pratylenchus and Pratylenchoides were bigb in some locations. Soil pbysicocbemical properties were investigated for tbeir relationship to nematode distribution. Tbere was a sligbt positive correlation of P. thornei and clay content; conversely, there was a significant negative correlation of P. negiectus witb clay and a positive correlation witb sand. Electrical conductivity (EC) was positively correlated witb P. neglectus. Nematodes in tbe genera Heiicotylenchus, Paratylenchus, Trophurus and Tylenchorhynchus were only recorded at low population densities in tbe sampled area. By contrast, nematodes in tbe genera Aphelenchus, Apheienchoides, Dityienchus, Dorylaimus, Tylenchus and bacterivorous genera bad relatively bigb populations. Total free-living nematodes were positively correlated witb EC and zinc (Zn) concentration. Tbe Zn content of soil was generally at a level deficient for plant growtb.

IntroductionTurkey is an important dry onion producer and the seventh leading producer after China, India, USA, Iran, Russian Federation, and Egypt with a total production of 1.8 × 10 6 t in 2012 (TÜİK, 2014). Onions are produced in many provinces of Turkey. Karaman Province, which includes terrestrial and Mediterranean mountain climate properties (Karaman Çevre ve Orman Müdürlüğü, 2007), contributes 12,657 t of dry onions or 0.0131 t/ha (2012 data;TÜİK, 2014). Onions are produced in an approximately 1000-ha area in central villages and in the Ermenek and Ayranci districts of Karaman (2012 data;TÜİK, 2014).Plant-parasitic nematodes cause 8.8%-14.6% crop losses annually and total economic yield losses of 100-157 billion US dollars worldwide (Nicol et al., 2011). Ditylenchus spp., Tylenchus spp., and Paratylenchus spp. were found most frequently and abundantly in the onion-growing areas of Karaman (Dikici and Yavuzaslanoglu, 2012). The stem and bulb nematode Ditylenchus dipsaci Kühn (Tylenchida: Anguinidae) is the most important nematode pest in a wide range of plants, including onion, garlic, hyacinth, narcissus, and tulip, particularly in temperate regions (Potter and Olthof, 1993;Tenente, 1996). Moreover, the European and Mediterranean Plant Protection Organization (EPPO) has placed D. dipsaci as no. 174 on the A2 list of phytosanitary categorization, which is distributed locally in EPPO countries, and it is regulated as a quarantine pest (EPPO, 1997).Crop losses differ depending on the initial nematode infection level in host plants. Sturhan and Brzeski (1991) reported crop losses of 60%-80% in heavily infected fields. Mennan ( 2001) reported that D. dipsaci caused a 65% yield loss in onion in the Suluova district of Amasya.

Nematode resistance and tolerance reactions of 28 onion cultivars grown commercially in Turkey to stem and bulb nematode were studied at 20 °C, 70% RH and 16:8 h L:D photoperiod in growth chamber with 10 replications and at 27±4 °C and 16:8 h L:D photoperiod in greenhouse with 3 replications, respectively. Ditylenchus dipsaci multiplied in all cultivars tested. The lowest multiplication was determined in cv. Valenciana from Atatürk Horticultural Central Research Institute with 91 nematodes/pot and a multiplication rate of 0.5. Plant height of cultivars were significantly different in the first tolerance experiment with having an average plant hight of 33.5 and 103.1 mm for inoculated and inoculated plants, respectively (P<0.05). Onion shoot diameter was statistically greater in nematode inoculated plants with 7.4 mm than inoculated plants with 6.0 mm in second tolerance experiment (P<0.05). Plant weight was not found different in both tolerance experiments with nematode inoculation. There was not any statistically difference among cultivars for plant height, plant diameter, plant weight and nematode multiplication in the experiments. Plant diameter for cv. Betapanko in first tolerance experiment and plant height for the Banko type onion in the second tolerance experiment sustained significant negative correlations with nematode numbers. Plant weight for cv. Biotek Boran in second experiment, plant height for cv. Taraz in second experiment and, plant diameter and plant weight for cv. Taraz in first tolerance experiment sustained significant positive correlations with nematode multiplication. It could be recommended that Valenciana could lower nematode multiplication and Biotek Boran and Taraz could maintain a better plant development in nematode infested onion growing areas.

Ditylenchus dipsaci is a species complex including diploid and polyploid individuals. The onion race of D. dipsaci is a sensu stricto group and has a wide range of host spectrum. Identification of the D. dipsaci onion race is difficult using morphological and morphometrical methods. Species specific primers are mostly used in molecular approaches for identification of D. dipsaci populations. Fifty one morphologically selected Ditylenchus spp. populations from onion production areas in Turkey were subjected to molecular identification using four D. dipsaci species specific primer sets (PF1-PR1, PF2-PR2, DdpS1-rDNA2, DitNF1-rDNA2, H05-H06) targeting 5.8S and 18S rDNA, ITS1 and flanking ITS regions. Thirty nine percent of the nematode samples were positive with four primers tested, while four of the nematode samples gave specific bands with H05-H06 primers. Ditylenchus dipsaci sensu stricto was identified with specific primer sets in Adana,