**6. Success stories of germplasm utilization in Australian cotton improvement**

#### **6.1. Disease resistance**

As mentioned previously, bacterial blight was the most important disease in Australian cotton when the modern Industry first started with estimated yield losses up to 20%. Bacterial blight resistance was introduced to the CSIRO breeding program from Tamcot SP37, which supplied the B2, B3 and B7 genes [97]. The first blight resistant cultivar was released in 1985 and had rapid adoption. Importantly, this cultivar also had high yield, high gin turnout, wide adapta‐ tion and the okra leaf trait. This cultivar was selected from a large breeding population that was initially developed in 1974, which demonstrates the considerable lag from introduction of a trait to commercial release.

trait has been associated with increased numbers of plant bugs (*Lygus spp.*), mirids (*Creontiades spp.*) and cotton flea hoppers (*Pseudatomoscelis seriatas*). The yield effect of OL has been reported

Australian Cotton Germplasm Resources http://dx.doi.org/10.5772/58414 25

In the 1970s, the CSIRO breeding program at Narrabri was focused on increasing yield, quality and disease resistance, but also had a strong commitment to increasing resistance to insects. To this end, significant effort was allocated to the OL trait, with large breeding populations developed as well as considerable research into the effect on yield. It was concluded that while there was a small (5%) reduction in yield potential, under commercial production systems with normal levels of insect damage there was no significant yield difference between OL and normal leaf (NL) near-isogenic lines. In addition, in situations where some insect damage was permitted, OL out-yielded NL by up to 10% [6]. A similar observation was reported by Brook *et al.* [98] who stated that OL was infested with fewer pests than NL and suffered less damage from a given density of pests and the reduction in yield caused by pest damage at high yield

Thomson [6] reported the reasons why OL has been so successful in Australia when it has either largely failed or been ignored in other countries. As mentioned at the beginning of this chapter, the modern cotton industry is relatively new in Australia, only being a significant industry since the early 1960s. Because growers did not have a long tradition of growing cotton, they had no fixation on what a cotton plant should look like – their concern being solely on performance. The absence of a yield penalty in commercial production systems as mentioned above was also a significant factor in the adoption of OL cottons. Importantly, the Australian OL cottons were not developed by backcross as most other studies have reported (e.g. [99-101]. Rather, as reported by [6], they were developed by hybridisation using wide crosses and large populations with heavy selection pressure for yield performance. Research, together with commercial experience demonstrated the host plant resistance of OL to spider mites and *Helicoverpa* as well as improved insecticide efficacy probably due to better spray penetration in the crop canopy. Lastly and of significant importance, the first OL cotton released for commercial sale (Siokra 1-1) was also the first cultivar in Australia to be completely resistant to bacterial blight. This was undoubtedly a significant factor in its rapid acceptance by growers.

The challenge of maintaining genetic diversity in this era of fierce intellectual property protection and commercial reality is significant. One of the ways this is being addressed in the CSIRO breeding program is by developing diversity through utilisation of the genetic resources outside the cultivated tetraploids. This can mean using landrace cottons, but more importantly evaluating traits in the secondary germplasm pool (A and D genomes) and accessing those traits by developing synthetic tetraploids. This approach to diversity requires considerable effort and a long-term vision; however, it does have the potential to pay sub‐ stantial dividends in unlocking traits previously unavailable from within elite cultivars. To justify this approach, there generally must be present a crucial trait of commercial importance

not available in existing germplasm and/or forward-looking funding.

as being anywhere from negative to positive and is thus far from clear cut.

levels was less in OL than NL.

**7. Conclusions**

Verticillium wilt continues to be an import disease for the Australian industry. However, there have been major successes in developing cultivars with good resistance to this disease. A major breakthrough was the release of Sicala V-1 in 1991, the first CSIRO cultivar with significant resistance, and its higher yielding replacement, Sicala V-2 in 1994. These cultivars show greatly reduced levels of infection, less severe symptoms and higher yields compared to their predecessors and have transformed those regions where Verticillium wilt severely limited yields. None of the parents of the population these cultivars were developed from had significant resistance to Verticillium wilt and would generally have been considered suscep‐ tible. The source of resistance has therefore been considered to be the result of additive effect of alleles at multiple loci. Molecular techniques may one day be able to determine the source and composition of this resistance.

After Fusarium wilt was first identified in Australian cotton in the early 1990s, new cultivars were required to allow viable production in the regions where the disease was present. It was quickly established that there was cultivar variability for resistance and a few cultivars did show some degree of resistance (Sicot 189, Delta Emerald). Most other commercial cultivars were very susceptible to the disease and had survival rate of close to zero in some situations. An initial screen from the CSIRO germplasm collection in 1994 tested 36 *G. hirsutum* genotypes from the USA, South America, Africa, Asia and Europe. As with the local cultivars, most were susceptible, but a few genotypes showed field survival up to 50% higher than the best Australian cultivars. Over the next few years over 200 genotypes from the collection were evaluated, with material from countries/regions/programs that showed promise in the early evaluations targeted. In 2004 a new cultivar, Sicot F-1, was released from the CSIRO breeding program which had at least twice the resistance of the best cultivar in 1994. Progress in breeding for improved Fusarium resistance continues, with the initial sources of resistance being derived from Indian and Chinese *G. hirsutum* parents and more recently, improved resistance from *G. hirsutum* and *G. barbadense* landrace cottons.

The key to the breeding success of soil borne fungal diseases has been the availability of screening sites with high levels of inoculum and therefore the opportunity for selection of reduced disease incidence and symptoms, together with high yield. The germplasm collections have been invaluable in accessing material for evaluation of resistance.

#### **6.2. Commercial utilisation of the okra leaf mutant**

The okra leaf (OL) mutant of upland cotton is characterised by deep lobing and some reduction in leaf area. This L2 o allele is partially dominant over the normal leaf (l2) shape. Although considerable research has been done on the effects of this leaf shape, only in Australia have OL cottons been grown on a major scale, accounting for between 40-60% of Australian seed sales between 1987 and 1993. As reviewed by Thomson [6] the published benefits of OL include host plant resistance to a range of insect and mite pests (boll weevil *Anthonomus grandis*, whitefly *Bemisia tabaci*, spider mites *Tetranychus spp.* and bollworms *Heliothis* and *Helicoverpa spp.*), earlier maturity, increased water-use efficiency and reduced boll rot. However, the OL trait has been associated with increased numbers of plant bugs (*Lygus spp.*), mirids (*Creontiades spp.*) and cotton flea hoppers (*Pseudatomoscelis seriatas*). The yield effect of OL has been reported as being anywhere from negative to positive and is thus far from clear cut.

In the 1970s, the CSIRO breeding program at Narrabri was focused on increasing yield, quality and disease resistance, but also had a strong commitment to increasing resistance to insects. To this end, significant effort was allocated to the OL trait, with large breeding populations developed as well as considerable research into the effect on yield. It was concluded that while there was a small (5%) reduction in yield potential, under commercial production systems with normal levels of insect damage there was no significant yield difference between OL and normal leaf (NL) near-isogenic lines. In addition, in situations where some insect damage was permitted, OL out-yielded NL by up to 10% [6]. A similar observation was reported by Brook *et al.* [98] who stated that OL was infested with fewer pests than NL and suffered less damage from a given density of pests and the reduction in yield caused by pest damage at high yield levels was less in OL than NL.

Thomson [6] reported the reasons why OL has been so successful in Australia when it has either largely failed or been ignored in other countries. As mentioned at the beginning of this chapter, the modern cotton industry is relatively new in Australia, only being a significant industry since the early 1960s. Because growers did not have a long tradition of growing cotton, they had no fixation on what a cotton plant should look like – their concern being solely on performance. The absence of a yield penalty in commercial production systems as mentioned above was also a significant factor in the adoption of OL cottons. Importantly, the Australian OL cottons were not developed by backcross as most other studies have reported (e.g. [99-101]. Rather, as reported by [6], they were developed by hybridisation using wide crosses and large populations with heavy selection pressure for yield performance. Research, together with commercial experience demonstrated the host plant resistance of OL to spider mites and *Helicoverpa* as well as improved insecticide efficacy probably due to better spray penetration in the crop canopy. Lastly and of significant importance, the first OL cotton released for commercial sale (Siokra 1-1) was also the first cultivar in Australia to be completely resistant to bacterial blight. This was undoubtedly a significant factor in its rapid acceptance by growers.
