Important Plant Parasitic Nematodes in Urban Agricultural Areas.

Urban agriculture is a variety of agricultural activities primarily carried out within the city limits. It is defined as 'plant and animal breeding in and around the city'. Urban agriculture can be at the household level or in the form of commercialized agriculture. Being self-sufficient is increasingly important in terms of food scarcity and safe food supply in the world. Therefore, epidemics, disasters, wars, etc. What such situations indicate is that the "agricultural sector" has an important place in the economic policies of countries (Akın et al., 2020). Urban agriculture’s scope includes food production in mass housing, small-density urban farms, land sharing, beekeeping, roof gardening, restaurant gardens, school gardens, food production in public spaces, private gardens, and balcony and window vegetable cultivation. Especially edible vegetables, flowers, medicinal and aromatic plants, and various vegetables constitute the product pattern of this sector. In terms of environmental quality and ecosystem, urban agriculture contributes to the sustainability of the city by maintaining biodiversity, reducing heat islands and carbon emission formation in cities, increasing the air quality of the town, and evaluating urban wastes and wastewater (Tandoğan & Özdamar, 2022). Urban gardens, established with intense labor, expense, and enthusiasm, need protection against many diseases and pests. Plants and animals in production areas close to these areas are more susceptible to diseases and pests, mainly due to the polluted nature of cities and decreased species diversity. Just like humans and animals, plants also have many diseases and pests. For this reason, to protect the products produced, it is necessary to have information about agricultural pests that may occur in the grown plants. There is a possibility of contamination of the same plants with the same diseases and pests in urban agriculture, as in the cultivated plants grown in the field and greenhouse agricultural areas. Nematodes are essential among many pests that threaten crop plants in agricultural areas. Nematodes can live in any environment where plants are present. These creatures appear as a factor limiting plant production in urban areas as well as in rural areas. Knowing nematodes that harm plants is essential to maintain the quantity and quality of the products produced in urban agriculture.

Nematodes are known as the most abundant invertebrate branches in the world. They are in the Animalia realm. They are roundworms in the Pseudocoelemata (animals with a false body cavity) subset of the suborder Eumetazoa. The word nematode, which has its root in Latin, means like a rope. It means 'parasitic maggot with filamentous structure' in Turkish. These creatures are animal organisms that can live harmoniously in very different ecological conditions. They can interact differently (parasitic, mutualistic, etc.) with other living things in their environment. Nematodes are also grouped as parasites, predators, or free-living with different feeding behaviors. Thanks to these nutritional characteristics, they can adapt to very different habitats. They can live in or on salty or fresh waters, moist soils, peat, dead and living people, animals, or plants. They generally like wet environments. Most of them die in dry conditions. In addition, some can live even in extreme environments such as deserts or glaciers. Some tropical nematodes enter anhydrobiosis in anhydrous and arid conditions and can survive even at soil temperatures of up to 50 °C. About 50% of nematodes live in the seas. 15% of these creatures live in vertebrates or invertebrates (including humans), and 10% in plants. The remaining 25% live freely (Kepenekci, 2012). Free-living ones feed on protozoa, fungi, bacteria, or other nematodes.

Records of nematodes living parasitically in humans and animals date back to ancient times. The first zoological record of the human body belongs to the Guinea worm, whose scientific name is Dranunculus medinensis (L.) (Camallanida: Dracunculidae). Afterward, it is possible to find records of many nematodes that harm humans or animals. The first record of nematodes that are harmful to plants belongs to the wheat gal nematode, which was found in 1743 by a scientist named Needham and whose current name is Anguina tritici (Steinbuch) (Tylenchida: Anguinidae) (Thorne, 1961; Siddigi, 2000). Studies on plant parasitic nematodes have been with the invention of the microscope, and taxonomic studies have progressed rapidly with the development of microscopes over time.

Nematodes are generally non-segmented and bilaterally symmetrical creatures with two nested body layers (Decraemer & Hunt, 2006). The largest of these creatures, which vary in size, can reach up to 2 meters. Those that feed on plants are usually microscopic. Although their bodies often appear threadlike, the females and males of some species may have different body shapes (dimorphism) from each other. Females can be shaped like a bag, sphere, pear, or lemon in these species. They are generally 400 µm-5 mm long, covering their body surfaces with soft, flexible skin (cuticle). While it is possible to see the large ones with the naked eye, it is impossible to see the small ones. Nematodes are colorless but can appear green, brown, or black depending on the plants they feed on. They have no eyes and no legs. They move by crawling. Those that feed on plants have a stinging needle called a stylet inside their mouth. In this way, they can pierce plants and feed on plant tissues.

Respiratory and circulatory organs are absent in nematodes. The muscular, excretory, and nervous systems comprise elementary cell groups. Digestive and reproductive systems are better developed than other systems.

Reproduction in nematodes occurs sexually, parthenogenetically, or by hermaphrodism. Depending on the ecological conditions, both sexual and parthenogenetic reproduction can occur in some species. Their mobility is limited.

Nematodes that cause damage by feeding on plants are called plant parasitic nematodes. Plant parasitic nematodes can also live in all environments where plants are present. Plant parasitic nematodes can move up to 1 meter a year with their movements in the soil. However, they can be passively transported from one crop production area to another by irrigation water, infected plants or soil, animals, people, or agricultural tools. They can also spread to distant areas by soil transport with strong winds. They can move vertically depending on soil temperature and soil moisture. During the dry seasons, they move from the soil surface to the depths. If the moisture on the soil surface increases, they move upwards again. They are usually found between 10-30 cm from the soil surface. However, depending on the root depth of the host plants, they can go down to 6 meters.

Plant parasitic nematodes have different biological periods. These are the egg, larva, and adult stages. The first larval period is spent in the egg. The larvae hatching from the eggs in the second period pass into the soil. They find their hosts by root secretions of nearby plants. It usually enters the plants from a region close to the root tip or starts to feed near the root tip. Feeding continues until adulthood. Plant parasitic nematodes become adults after four larval stages. They molt as they pass from one larval stage to the next. The majority of these nematodes cause damage to the roots of plants. Very few of them cause damage to plant parts such as leaves, flowers, or stems (Hunt et al., 2005). It physically damages the plant cell wall primarily by means of stinging-sucking needles called stylets. Then, they disrupt the structure of the cell wall by means of their pectolytic and cellulitic enzymes (Jaubert et al., 2002). By means of these methods, plant parasitic nematodes feed effectively or can move between cells (Karssen & Moens, 2006). As a result of their feeding, they cause deformations in the stem cells, thus preventing the transmission of nutrients taken from the soil. Due to some toxic substances in their secretions in their styles, they can damage the host plant cells and even lead to the death of the cells (Prot, 1985; Abad et al., 2003; Karssen & Moens, 2006; Schomaker & Been, 2006). Plant parasitic nematodes cause significant damage to plants indirectly as well as directly. Due to the wounds, they cause on the roots, they cause some soil-borne disease agents (fungal, bacterial) to enter the host plants. Some nematodes, on the other hand, carry many plant virus diseases that do not have chemical treatment for plants during their feeding. Such nematodes are called virus vector nematodes. The damage (secondary) they cause in this way can sometimes be much more than the damage they cause through their direct feeding.

Injury caused by plant parasitic nematodes is often similar to symptoms caused by inanimate environmental conditions or other disease agents. For this reason, they are often confused with each other. Soil and plant samples should be taken when symptoms such as wilting, nutrient deficiency, or growth retardation occur in plants. The samples should be analyzed in the laboratory in terms of nematodes.