In areas of protected cultivation carried out intensively, without crop rotation or fallow, melon planting has been affected by several diseases (Müller; Vizzotto, 1999, Charlo et al. 2011), among which nematodes are important pathogens, which in high infestations cause significant losses. Among the species found in melon cultivation, the most notable are root-knot nematodes, Meloidogyne incognita e Meloidogyne javanica; the reniform nematode, Rotylenchulus reniformis; and the root lesion nematode, Pratylenchus brachyurus.
There are several species of nematodes in the genus Meloidogyne associated with the roots of the melon tree, highlighting: M. javanica, M. hapla e M. arenaria. However, the most common species that causes the greatest damage is the M. unexplained, whose losses can reach up to 100% of crop production. This species is the causal agent of the disease known as meloidoginosis.
In Brazil, its presence has been observed in most melon growing areas, mainly in the Northeast region. Losses arising from the pathogen can limit production, reaching levels of up to 100%. The nematode has a wide range of hosts, including cucurbits, solanaceae, legumes and other plants of economic interest.
The characteristic symptom due to nematode infestation is the presence of galls on the roots (Figure 1). The roots of infected plants increase in size (hypertrophy) and quantity (hyperplasia). Symptoms of yellowing and premature fall are observed on the leaves. Infested plants are uneven in size, bushy, less vigorous and wither in the hottest hours of the day. The leaves and fruits produced by symptomatic plants are reduced in size and have a reduced percentage of sucrose. As a result of the damage caused by the nematode, there is a reduction in crop production.
Nematodes damage roots and encourage the penetration of fungi and bacteria, which contribute to the development of disease complexes. For example, the interaction of Didymella bryoniae on Meloidogyne javanica in areas infested by root-knot nematode in melon crops, it causes drought in the aerial part of the plants (Figure 2).
The root-knot nematode is active throughout the year in hot climates and moist soils. In colder climates, the life cycle is longer. Root-knot nematode species are obligate parasites of roots and underground stems. They are mobile in the soil, and the worm-shaped or juvenile stages of development of the second stage (J2) are the life forms that infect the melon roots present in the soil.
In response to the introduction of substances produced by the esophageal glands of juveniles into the plant's root tissues, there is an increase in the size and number of parasitized root cells, which results in thickening called “gall”. In the adult stage, the male usually leaves the root and no longer parasitizes the plant. Adult males of these nematodes are worm-shaped and do not feed. The female continues its development until it assumes a globose and pyriform shape and, subsequently, produces a mass of eggs that generally remain outside the root, with the possibility of being seen with the naked eye.
This mass contains, on average, 500 eggs to a thousand eggs surrounded by a gelatinous substance that protects them against desiccation and other unfavorable conditions. In certain situations, such as high temperatures and a highly susceptible host, the number of eggs produced in this mass can exceed two thousand units. Inside each egg, the formation of the first stage juvenile (J1) will occur, which undergoes ecdysis and transforms into the second stage juvenile (J2), still inside the egg. This represents the infective form that hatches from the egg, goes into the soil or directly infects another root, going through three more ecdyses until reaching the adult stage, thus completing the cycle in around 21 days to 45 days, depending on climatic conditions and of the nematode species involved, with the possibility of being completed in up to 70 days in winter.
The root-knot nematode moves a short distance, randomly between soil particles, until it finds the root system. Spread occurs under adequate temperature and soil moisture conditions. Sandy soils are the most suitable for the spread of the nematode. Long-distance dissemination is carried out through irrigation water or rain, the transport of soil particles or roots, tools, machines and agricultural implements. In the absence of the host, the pathogen can survive in the form of eggs and juveniles that are found in the soil or in the roots of weeds or remaining crop plants. As an example of root-knot nematode host weed plants, Maria pretinha (Solanum americanum), joá-de-Capote (Nicandra physaloides), false milkweed (emilia fosbergii), juá bravo (Solanum sisymbriifolium), caruru (Amaranthus hybridus), breaks horse (Solanum aculeatissimum), São Caetano melon (Momordica), among others. The life cycle of the root-knot nematode is represented in Figure 3.
The adoption of different integrated management measures should be the most used strategy, so that the root-knot nematode population remains below levels of economic damage. The most efficient preventive measures consist of preventing the entry of root-knot nematodes into melon growing areas, selecting areas for planting that are free of the pathogen and, above all, using healthy seedlings. Other management measures for root-knot nematode include fallowing with deep plowing and successive harrowing, fallow, elimination of crop residues, cleaning of tools, machines and agricultural implements, planting of antagonistic plants, crop rotation, biological control and the use of chemical control as a last resort. For root-knot nematode management, rotation is a complex activity, except when non-host plants are used, such as grasses (resistant cultivars Brachiaria, corn and millet). It should be noted that, to date, there are no commercial melon cultivars resistant to these nematodes.
The planting of antagonistic plants causes a reduction in nematode population levels. Crotalaria (Crotalaria spectabilis, Crotalaria juncea), marigold (Tagetes patula, Tagetes minuta, Tagetes erecta and mucunas (Mucuna lands) are examples of antagonistic plants that are successfully used to control nematodes. It is worth highlighting the fact that black velvet (Mucuna lands) has proven efficacy for M. unexplained, but not for M. javanica. For the control of species of Pratylenchus only the planting of Crotalaria spectabilis and marigold.
Melon cultivars grafted onto nematode-resistant cucurbit rootstocks is an alternative management practice for root-knot nematodes (Thies et al. 2010; Ito et al. 2014). Interest in this technique has been growing around the world, due to the advantages of resistance and/or tolerance to high and low temperatures in addition to nematodes (Kubota et al, 2008; Lee et al. 2010). Hybrid pumpkins (Cucurbita maxima Duchesne × Cucurbita moschata Duchesne) are widely used as melon rootstocks, being highly resistant to fusarium wilt, whorl and gummy blight (Guan et al. 2012; Louws; Rivard; Kubota 2010). However, they are susceptible to root-knot nematodes and can have adverse effects on the fruit quality of some melon cultivars (Davis et al. 2008; Sakata; Ohara; Sugiyama, 2008). When using grafted seedlings, for the investment made to have a financial return, the rootstock must not only be resistant to diseases, but must also have compatibility, maintaining the productivity and characteristics of the cultivar used as graft (Jang et al. 2012; Guan; Zhao, 2014). Chemical control of meloidoginosis must be carried out with chemical products registered by the Ministry of Agriculture, Livestock and Supply (Mapa). Regarding biological control, several organisms present in the soil are nematode parasites, with an emphasis on fungi and bacteria, used as an incremental technology in the integration of other control measures. For example, the bacteria Pasteuria penetrans is an obligate parasite of several species of Meloidogyne.
Rotylenchulus reniformis, commonly known as kidney nematode due to the morphological appearance that the female has as an adult in the form of a “kidney”, is a nematode that is of great importance for melon cultivation.
The reniform nematode has a wide range of hosts, such as cowpea (Unguiculata vine), pigeonpea (Cajanus cajan), banana (Muse paradisiaca), pineapple (Pineapple comosus), cotton (Gossypium hirsutum), castor (Ricinus communis), cucumber (Cucumis sativus), pumpkin and zucchini (Cucurbita moschata), watermelon (Citrullus lanatus), sweet potato (Ipomoea batatas), coriander (Coriandrum sativum), okra (Abelmoschus esculentus), soy (Glycinemax), passion fruit (Passiflora), gherkin (cucumis anguria), tomato (Solanum lycopersicum) and weeds belonging to the Malvaceae and Cucurbitaceae families.
The reniform nematode causes damage to the root system, interfering with the plant's nutrient absorption. Heavily infected plants develop symptoms of deficiency of nitrogen, potassium, manganese and other nutrients due to limited absorption by the roots. Furthermore, they show reduced growth and wilt under water stress, chlorosis and irregular spots. The consequence of damage caused by the nematode is a reduction in the yield and quality of melon fruits.
The female of Rotylenchulus reniformis is a sedentary ectoparasite, which parasitizes the external surface of the melon tree roots. All life forms of Rotylenchulus reniformis as juveniles, immature males and females survive in the soil. The male is small and does not parasitize the roots. Juveniles hatch from the egg and then, in the form of J2, move on the ground and undergo three more ecdyses before feeding. After the last ecdysis, immature wormlike females find the roots and parasitize them. Over time, the female's body swells and takes on the appearance of a kidney. Initially, during penetration, immature females cause destruction of epidermal cells, resulting in small necrotic lesions. With the movement of its anterior region through the cortical parenchyma, cell death occurs and the immature female reaches the endodermis and pericycle where it will establish its site of infection in the phloem. Between five and ten cells of the root tissue around its anterior region ('head') will be affected by substances that are produced by its esophageal glands. The female will remain at the feeding site until she becomes an adult female. Over time, phloem necrosis and collapse of the cortex region occur, resulting in reduced growth of the root system and a consequent reduction in plant growth. The female lays an average of 50 eggs to 100 eggs in a mucilage that is attached to her posterior region, external to the root. The egg-to-egg life cycle is completed in approximately 24 days to 29 days, depending on the host species, soil type, and environmental conditions such as soil temperature and humidity (Figure 4). mobile stadiums R. reniformis they can survive in the soil for at least six months at temperatures ranging from -4° to 25°.
The reniform nematode is capable of persisting for long periods in the soil without the presence of the host. Therefore, measures such as fallowing or fallowing are not recommended for control. Despite the wide range of hosts, crop rotation is an interesting measure for managing R. reniformis. Non-host plants, such as sunn hemp and marigold, can help reduce population levels of this pathogen. To date, no commercial melon cultivars resistant to the reniform nematode have been identified. Regarding chemical control, to date, there are no nematicides registered on the Map for the management of reniform nematodes in melon plantations.
It is a nematode that also causes important reductions in melon production.
The first report of root lesion nematode causing natural infections in melon fields was in the Assu-Mossoró Polo region. More than 300 plants from different botanical families have been reported as hosts of Pratylenchus spp. The root lesion nematode (Pratylenchus spp.) has been reported causing severe damage to several crops of economic importance, such as okra, pineapple, soybeans, beans, cotton, corn, especially in the cerrado region.
Symptoms caused by nematodes of the genus Pratylenchus They are not specific and can be easily confused with those caused by other pathogens or nutritional deficiencies. However, the main symptom lies in the presence of intense dark lesions (necrotic) on the roots and radicelles of parasitized plants.
Sick plants usually appear as lumps in the field. Fungi and bacteria can penetrate these lesions, increasing damage to the roots and, consequently, causing rot. Furthermore, they can cause drastic reduction in growth and delay in their cycle. In the aerial part, stunted growth and the presence of chlorotic leaves are observed, and a consequent reduction in the yield of the melon tree.
Currently there are more than 70 species of Pratylenchus with a wide range of hosts and widespread distribution in different regions of the world. In melon cultivation, the most important species is P. brachyurus.
They are migratory endoparasites that cause damage to roots due to feeding, active movement and the release of enzymes and toxins in the root cortex. The first ecdysis of Pratylenchus occurs inside the egg, from where the second stage juvenile emerges. All stages of development are active and vermiform (Figure 5), being able to penetrate the melon roots, from where they continuously migrate in the intra- and intercellular tissues and reproduce, reaching high population levels.
Nematodes of the genus Pratylenchus They remain migratory throughout their life cycle and move actively in the soil until they find the melon roots, where they penetrate and migrate into the root cortex, being able to return to the soil. Females lay their eggs singly or in groups in the soil or on roots. Each female produces, on average, approximately 80 eggs to 150 eggs throughout her life.
The damage caused by species of the genus Pratylenchus are distinct when compared to those caused by root-knot nematodes, basically due to differences in their life cycles. Damage and population levels vary greatly, from 0,05 nematodes/cm3 to 30 nematodes/cm3 of soil and life cycle of Pratylenchus three to four weeks. Hundreds of weeds are hosts to root lesion nematodes, mainly within the grass family, which can contribute to the maintenance and increase of population levels in the field.
One of the main factors responsible for the distribution and dissemination of nematodes of the genus Pratylenchus It's the texture of the soil. Soils with a sandy or medium texture generally favor most species in the genus. Another factor that favors the life cycle of root lesion nematodes is soil moisture, where 70% to 80% of field capacity represents an optimal condition for various nematode activities.
Control of the pathogen must be carried out through the integration of several preventive practices, in order to avoid the entry of the nematode into the area, and control to reduce nematode population levels in areas already infested, as once infested, it is impossible to eradicate the pathogen in the area.
Regarding crop rotation, there are few species options for this practice due to its wide host range. Sunn hemp, especially Crotalaria spectabilis, are good options for use in crop rotation, as they reduce nematode population levels after a cultivation cycle.
It is important to mention that the concomitant occurrence in the same area of root lesion nematode (Pratylenchus brachyurus) and root-knot nematode (Meloidogyne spp.) makes cultural management very difficult in relation to crop rotation, since both genera are polyphagous. Furthermore, Pratylenchus feeds preferentially on grasses, mainly corn and millet, which makes cultural management difficult, given that the recommendation of crop rotation to Meloidogyne prioritizes the use of species from this family.
For areas known to be infected, fallowing is recommended, which consists of keeping the soil without host plants or any type of vegetation, with the soil being disturbed by plowing or harrowing at intervals of 15 days to 20 days for two months, which constitutes an excellent management measure. It is worth mentioning that excess nitrogen fertilization and irrigation can increase plant damage. Pratylenchus. To date, there is no mention in the literature of the use of grafting as an alternative for managing root lesion nematode.
Jadir Borges Pinheiro, Raphael Augusto de Castro e Melo, Alexandre Augusto de Morais, Embrapa Hortaliças
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