Article: Matshediso Motshabi
Plant parasitic nematodes (PPNs) are thread-like, colourless, microscopic worms that are widely acknowledged as serious agricultural pathogens that, if left unchecked, results in severe crop losses. Farmers commonly lose a lot of money each year due to agricultural losses brought on by PPNs, but they can also cause issues in urban areas by harming ornamentals, residential gardens, and turf grass. Nematodes frequently produce harm so subtle that it goes unrecognized or is attributed to other sources, which makes it difficult for humans to recognize the losses they inflict. Nematodes, which are the second phylum behind Arthropoda in terms of species diversity, are thought to number over a million. About 50% of all known nematodes are found in marine habitats, and 25% are found in soil or freshwater. They eat bacteria, fungi, other animals that break down organic debris, small invertebrates, or other decomposer species. About 15% are parasitic to humans, domestic animals, wild animals, and insects. Only 10% of known nematodes are plant parasites.
While female plant nematodes are typically cylindrical in shape, depending on the species, they tend to shed their worm shape as they age and instead take on a round, pear, lemon, or kidney shape. PPNs go through six stages in their life cycle: egg, four juvenile stages, and adult. Most species of nematodes have both male and female members, however reproduction without males is frequent and certain species are hermaphrodite (produce both sperm and eggs). Depending on the nematode species and their environment, the majority of PPNs species produce between 50 and 500 eggs per female, although others can produce more than 1,000 eggs. The length of the life cycle varies greatly depending on the species of nematode, the host plant, and the environment temperature. The life cycle of many plant nematodes takes about four weeks to complete during the summer when soil temperatures range from 24 to 35°C.
PPNs pierce host cells with an oral stylet, a spearing tool that resembles a hypodermic needle, to feed on living plant tissues. In order to partially digest the contents of the host cell, certain plant nematodes inject enzymes into the host cell prior to feeding. The majority of damage that nematodes inflict on plants is connected in some manner to its feeding activity. Different plant nematode species have different feeding habits. Some are Ectoparasites, or soil-dwelling nematodes, which feed on the surface of root tissue by slipping their stylets into any accessible cells, e.g. stubby- root, needle and dagger nematodes. Nematodes that completely penetrate root tissue and feed there are known as endoparasites. Nematodes that are endoparasitic can be further separated into sedentary and migratory endoparasites. Endoparasites that migrate continue to move and eat as they pass through the tissue of plants. They are found in the soil but spend the majority of their lives in plant tissue. Sedentary endoparasites establish a long-term feeding location in the plant tissue. Root-knot, lance, and lesion nematodes are a few examples. Semi-endoparasite are nematodes that only partially enter plant tissue, leaving the back of their bodies protruding into the soil. Reniform nematodes are an example.
Root-knot, root-lesion, and maize cyst nematodes are PPNs that have a significant economic impact on grain crops in Botswana. The sedentary endoparasites known as root-knot nematodes (Meloidogyne spp.) spend the majority of their life cycle inside the root tissue, where they develop tiny galls on the roots. Stunting and/or yellowing of the leaves, as well as browning of the roots, are symptoms. Effective control requires an understanding of the exact species present because there are numerous species of root-knot nematodes that have a wide variety of host crops. Three of the four most prevalent Meloidogyne species attack maize. Adult females of the maize cyst nematode (Heterodera zeae) are pear- or lemon-shaped, pearly white in colour, and as the cyst develops, they turn dark brown. Its biology is influenced by temperature because it has a short life cycle in warm climates. The maize cyst nematode causes grain crops to become stunted, pale in colour, and with narrow leaves. Tassel development may be substantially delayed and stunting frequently occurs in patchy regions within the field. The most crucial factor in identifying H. zeae is the appearance of cysts on the root surface. Millet, oat, rice, sorghum, sugarcane, and wheat are other economic hosts. Pretylenchus zeae, a migratory endoparasite, is slender shaped throughout its adult stage. Stunting, wilting, premature yellowing of plants, and visible root lesions are just a few of the effects they cause.
Dagger, stem, stubby root, needle, lance, spiral, and ring nematodes are a few more minor PPNs that are sporadic. Crop rotation with non-hosts and the planting of cereal cultivars that are resistant to their infestation significantly diminish their populations, and are the most efficient ways to control all species of PPN. As most weeds are hosts for numerous nematodes and may serve as reservoirs for the current or following season’s crop, removing them is one of the other management practices. Fertilize in accordance with the results of soil tests since plants with nutrient deficiencies are more vulnerable to nematode damage. Nematicides containing the active components Carbofuran, Fensulfothion, Terbufos, and Ethoprop have been approved for use in Botswana to control nematodes on cereals. Before applying, please read the label carefully because some chemicals can be placed in a band or a furrow, while others are exclusively advised for a specific type of application. Happy farming!
