Meloidogyne enterolobii parasitizing cotton in Brazil

Cotton is a versatile and durable natural fiber with a long history of use in the textile industry. Cotton is the most important natural textile fiber in the world, representing around 25% of global fiber production.

Brazil is the world's second largest exporter of cotton, with an important share in Brazilian GDP. Among the main biotic agents that affect and reduce the productive potential of cotton plants (Gossypium hirsutum L.), phytonematodes stand out Meloidogyne incognita, Rotylenchulus reniformis, Pratylenchus brachyurus e Aphelenchoides besseyi.

Root-knot nematodes, genus Meloidogyne Göldi, 1887, is known as the most important for its wide worldwide distribution and for causing severe damage to a wide variety of agricultural crops. To date, only three species of root-knot nematodes are considered parasites of cotton plants, M. unexplained, physiological races 3 and 4 with common predominance in cotton and soybean succession areas, M. acronea, with a restricted occurrence reported only in southern Africa and M. enterolobii, a recently detected species in cotton, reported only in Brazil (Galbieri and collaborators, 2020) and in the USA (Ye and collaborators, 2013).

The first crop in our country to signal the occurrence of M. enterolobii (synonym of M. mayaguensis) was the guava tree (Psidium guava) in the 1990s. In 2001, Embrapa researcher Regina Carneiro – a reference in studies on taxonomy of Meloidogyne – and other authors, reported for the first time this nematode as the causal agent of the decline and death of guava trees, based on samples collected in the states of Pernambuco and Bahia. Later, the involvement of the fungus was proven Fusarium solani in the expression of symptoms observed in guava orchards. Currently, it is known that the nematode parasitizes an extensive list of hosts, including large crops (commodities), vegetables and fruit plants.

Infection begins when nematodes in the second stage (J2) juvenile stage penetrate the roots and develop into adults (mostly females). Females are pear-shaped and can reach up to 1 mm in length. They lay their eggs on the roots of host plants, in a gelatinous mass that helps them survive in the soil. Each female can produce more than 400 eggs that will continue the cycle.

The identified guava population was spread to other Brazilian states through the sale of guava seedlings contaminated by the nematode in the northeast region, where many commercial nurseries were located (Carneiro and collaborators, 2021). In guava roots infected by M. enterolobii a large number of galls are observed, in addition to necrosis. For the benefit of guava producers, Embrapa launched in 2019 the resistant rootstock 'BRS Guaraçá', a hybrid resulting from the crossing between Psidium guava e P. guineense, which showed resistance to different populations of M. enterolobii (Proceedings of the 38th CBN, n°78, Brazilian Congress of Nematology, Cuiabá-MT).

In soybeans, the nematode M. enterolobii was found for the first time in 2008 in the state of São Paulo by professor Jaime Maia's team, and more recently (2019) IMAmt researcher Rafael Galbieri detected the presence of this nematode overcoming the source of resistance M315 to M. unexplained, present in a resistant cotton cultivar. It is worth noting that the population of M. enterolobii of guava does not parasitize cotton, and is therefore suggested as a new breed of pathogen in recent studies carried out by the University of Brasília and Embrapa Cenargen (Souza, 2022; Verssiani, 2022).

In a recent study, different soybean cultivars were evaluated for resistance to two populations (races) of M. enterolobii (guava, cotton). One cultivar, BRS 7180 IPRO, was considered moderately resistant (MR) to both races; and BRS 7980 (MR) only for the guava breed. Both cultivars are resistant to M. javanica e M. unexplained (Verssiani and collaborators, 2023).

An important aspect of this nematode is its aggressiveness to cotton, which manifests itself in the formation of large galls on the roots of infected plants, compared to galls caused by other species of root-knot nematode.

We know that controlling nematodes with chemical or biological nematicides is not a guarantee of effective control. The best possible option is the development of cultivars with genetic resistance, when available, to ensure efficient management of the nematode. The current cotton seed market has five cultivars with resistance to M. unexplained. This is an inheritance governed by genes with a greater effect (oligogenic inheritance), determined by two QTLs (gene loci that express quantitative characteristics) located on chromosomes 11 and 14.

Meloidogyne enterolobii It is characterized by the ability to cause disease in cultivars carrying resistance genes to the main root-knot nematodes, in crops such as potatoes, sweet potatoes, tomatoes, peppers, cowpeas, soybeans and now cotton. In the case of cotton, even in cultivars with resistance genes to M. unexplained, the new variant of M. enterolobii is capable of infecting and multiplying.

In the USA, the population of M. enterolobii was found parasitizing cotton and soybeans in the state of North Carolina. In Brazil, the recent population variant detected in cotton is still restricted to the field, with four cases already confirmed. IMAmt researcher Rafael Galbieri presented reports of the detection of this new population of the pathogen during the 38th CBN.

Only a few crops of agricultural importance, such as corn, sorghum, wheat, oats, garlic, peanuts, castor beans, passion fruit and Crotalaria spectabilis, were reported as non-hosts or poor hosts of M. enterolobii. They can be recommended within management based on crop rotation in infested areas. No commercial cotton cultivar with resistance to M. enterolobii is available right now. There are also no registered nematicidal products available for the management of this nematode in large crops, such as cotton, soybeans and beans.

Prevention is the best alternative for nematode control M. enterolobii in cotton. To this end, it is important to: prevent the nematode from entering cotton growing areas, keep agricultural machinery clean before entering other plots, dispose of residual water from machine washing in a safe place, clean boots and shoes as they can be a source of inoculum for the nematode and use cover crops that are bad hosts to reduce its multiplication.

It is also important to highlight the need for periodic inspections of plantations for early detection of possible nematode outbreaks. If the presence of the nematode is suspected, the correct diagnosis can be carried out by a laboratory specialized in electrophoresis analysis of isoenzymes and molecular markers.

By Jorge Bleno da Silva Verssiani (Unicampo/BASF), Rafael Galbieri (IMAmt) and Juvenile Enrique Cares (UnB)

Article published in issue 293 of Cultivar Grandes Culturas Magazine.