Brasilia, 3-6 Setembro 2013

Sala Especializada 6 - Manejo dos nematoides no algodoeiro. OBTAINING AND USING TOLERANT AND RESISTANT GENOTYPES FOR NEMATODE MANAGEMENT IN COTTON Richard F. Davis U.S. Department of Agriculture, Agricultural Research Service, Crop Protection and Management Research Unit, P.O. Box 748, Tifton, GA 31793, USA.

The use of host plant resistance as a nematode management strategy in cotton (Gossypium hirsutum) has great potential. Using resistant cultivars is among the easiest management tactics for farmers to implement and it has a more consistent effect than other options. Planting a resistant cultivar requires no special equipment or calibration. Unlike nematicides, resistance suppresses nematode levels throughout the season and can provide some protection for a subsequent susceptible crop. Resistant cultivars are typically the most cost effective means of minimizing losses to nematodes. However, until recently, the greatest obstacle to developing highly-resistant cultivars has been the lack of a rapid and efficient method for selecting resistant genotypes in breeding programs. Documenting resistance relies on measuring nematode reproduction, although measuring the degree of root galling is sometimes used with M. incognita, but either of these processes are slow, labor intensive, and therefore relatively expensive. With the discovery of DNA markers linked to nematode resistance genes, cotton breeding programs have begun to place much more emphasis on developing resistant cultivars. Sources of resistance to Meloidogyne incognita and Rotylenchulus reniformis are available in Gossypium hirsutum and other Gossypium species. The first cotton germplasm with a high level of resistant to M. incognita, Auburn 623 RNR, was developed in the 1960s and released to breeders in 1970. It was developed by crossing two moderately-resistant genotypes and then selecting a transgressive segregant that had a much higher level of resistance than either parent. Several generations of improved germplasm have been derived from Auburn 623 RNR, with the most resistant lines developed through backcrossing the resistance genes into a susceptible line that had desirable characteristics. Attempts to breed for resistance without using backcrossing were generally unsuccessful. Although effective, the many generations needed for backcrossing to incorporate resistance generally results in plants whose yield and fiber quality lag those of the best susceptible lines. DNA markers linked to two resistance genes, one on chromosome 11 and one on chromosome 14, were first reported in 2006 with subsequent refinements in the years since. The identification of these markers has allowed breeders to use marker assisted selection to incorporate the resistance genes into new germplasm and cultivars much more rapidly and without the need for backcrossing. The development of resistance to R. reniformis has been much more recent. Several thousand accessions of Gossypium spp. were evaluated, and only moderate resistance at best was identified in G. hirsutum, but very high levels were identified in other Gossypium species. Although resistance in G. barbadense can suppress R. reniformis by 70-90%, resistance from other species suppresses reproduction by 95 to more than 99%. Introgressing genes from the tetraploid G. barbadense is achieved through simple, traditional crosses. Introgressing genes from other species, which are typically diploids, is much more difficult and involves the use of hexaploid bridging lines or tri-species hybrids. However, introgression of resistance to R. reniformis from multiple sources was successful and is now available in G. hirsutum germplasm. A high level of resistance to

Brasilia, 3-6 Setembro 2013

R. reniformis appears to typically be imparted by single genes, and markers for the various known genes are now available. However, because R. reniformis is less widespread and causes less damage to US cotton production than M. incognita, developing cultivars with resistance to R. reniformis appears to be a lower priority at this time. Growing cultivars with the same resistance genes year after year puts significant selection pressure on the nematodes to overcome that resistance. If a single source of resistance is relied upon continuously for too long, it is likely to eventually be rendered ineffective. This will be especially true for resistance to R. reniformis because the nematode reproduces sexually and the resistance is due to single genes. Progeny of M. incognita do not gain genetic diversity from sexual recombination and the resistance is multigenic. Many examples of pathogens overcoming host plant resistance suggest that the ability to identify and then combine or rotate unique resistance genes is important to prolong the effectiveness of the resistance. Ways to minimize selection pressure in cotton will be to rotate a resistant cultivar with a susceptible crop, either another plant species or a susceptible cotton genotype, or to rotate resistant cotton cultivars that contain different resistance genes.

The availability of DNA markers for nematode resistance genes provides a tool to help determine whether various genes are in fact unique. However, identifying candidates to evaluate will still rely heavily on large-scale screening of cotton germplasm collections. Many entries in germplasm collections have been previously evaluated, but in the search for sources of resistance, moderate resistance is often deemed inadequate and many moderately-resistant entries may have gone unreported. That is unfortunate considering that the very high level of resistance to M. incognita in Auburn 623 RNR was achieved by combining resistance genes from two moderately-resistant genotypes. A poor host can often provide as much rotational benefit to a subsequent susceptible crop as a non-host crop. Although the degree of yield suppression caused by nematodes should decrease as the level of resistance increases, even moderate levels of nematode resistance are useful for reducing damage. Rotating moderately-resistant cotton with susceptible cotton can provide a yield benefit to both crops, especially if nematicides are also used, but even a highly-resistant cotton cultivar is unlikely to provide nematode suppression beyond the first year of a subsequent susceptible crop. Highly-resistant cultivars may still benefit from a nematicide application in fields with the greatest nematode pressure. Resistant plants prevent the nematodes from completing their life cycle, but they do not prevent the nematodes from penetrating the roots of the plant, and sufficient numbers of nematodes entering the roots of a plant may be damaging even without subsequent reproduction if the plant exhibits a strong hypersensitive response. This has been shown to occur with the R. reniformis-resistant LONREN lines. Nematode tolerance in cotton is not as well documented or understood as resistance. Resistance and susceptibility refer to the effect of the plant on a nematode’s ability to reproduce. Tolerance and intolerance describe the degree of damage inflicted on the plant by the nematode. The genetics of nematode tolerance in cotton are unknown. Although tolerance in cotton has been reported, many suspected tolerant genotypes may be expressing moderate levels of resistance which result in the plant suffering lower levels of nematode parasitism. One possible mechanism of tolerance could be larger root systems in tolerant lines resulting in a reduced parasitic load (nematodes per gram root). Tolerance is an intriguing phenomenon that deserves further research.