Genetic characterisation of Striga gesnerioides and its control strategies in tobacco (Nicotiana tabacum L).

Abstract

Tobacco (Nicotiana tabacum) is an important contributor to the GDP and export revenue of Zimbabwe. One of the emerging threats to tobacco production is Striga gesnerioides, a root parasitic weed which belongs to the Orobanchaceae family. Striga gesnerioides infection in tobacco can cause up to 100 percent leaf yield loss. The objectives of this study were as follows: first, to determine genetic characterization of S. gesnerioides; second, to determine the impact of trap crops on the S. gesnerioides seed banks and S. gesnerioides emergence; third, to determine tobacco leaf yields in S. gesnerioides environments; and fourth, to evaluate tobacco varieties for the production of strigolactones using S. gesnerioides seed germination percentages and maximum germination distances from tobacco main roots in agar gel assays. For the first objective, specimens of S. gesnerioides plants were collected from parasitized tobacco and other host plants during the 2016/17 and 2017/18 growing seasons in Mvuma, Chegutu, Selous, Norton and Dotito areas in Zimbabwe. The specimens were identified to species level using morphological characters and further characterized using molecular techniques (modified CTAB method). For the second objective, trap cropping treatments were implemented at Mvuma and Chegutu in Zimbabwe in the 2016/17 and 2017/18 cropping seasons. At these sites, cowpea, bambara nut, groundnut, common bean, sunn hemp and tobacco, were grown for two years, and some plots were left to lie fallow for two years, followed by tobacco as a bio-test crop. These treatments were replicated three times in a randomized complete block design in the 2016/17 and 2017/18 cropping seasons. During these periods, the S. gesnerioides seed bank size was estimated after extracting seeds from 200g soil samples at the beginning and end of the field experiment, using the sucrose method. For the third objective, 15 tobacco varieties were grown in experimental plots at Mvuma and Chegutu in the 2016/17 and 2017/18 cropping seasons, with 3 replications for each tobacco variety arranged in a randomized complete block design. For the fourth objective, the agar gel bioassay was used to evaluate 15 tobacco genotypes for the production of strigolactones under laboratory conditions. The 15 tobacco varieties were replicated three times in a randomized complete block design in the laboratory. For the first objective, the DNA from specimens collected from Striga plants infesting tobacco in the three reported areas (Chegutu, Mvuma and Selous) were 100% similar. Parasitic plants collected on Richardia scabra in Dotito and Dalbergia melanoxylon in Mvuma, on the assumption that they were S. gesnerioides races were genetically different from the tobacco Striga and were identified as Sopubia lanata. For the second objective, it was established that growing trap crops for two seasons did not have significant (p>0.05) effects on S. gesnerioides seed banks at both Mvuma and Chegutu sites. However, bambara nut appeared to reduce S. gesnerioides seeds in the soil at both sites. Trap crops had significant effects (p<0.05) on S. gesnerioides counts at Mvuma. It was found that growing bambara nut, common bean, sunn hemp, groundnut and cowpea and letting land lie fallow for two years decreased the emergence of S. gesnerioides at Mvuma. In contrast, growing soyabean and tobacco for two seasons followed by tobacco had the highest number of S. gesnerioides counts at Mvuma. Trap crop effects were not significant (p<0.05) at Chegutu. However, growing soyabean and letting land lie fallow for two seasons had low S. gesnerioides counts, whereas groundnut had the highest S. gesnerioides at Chegutu. For the third objective, the tobacco varieties had no significant (p>0.05) effect on tobacco leaf yield for two seasons at Mvuma. However, tobacco varieties were significantly (p<0.05) different with regard v to leaf yield in the 2017/18 season at Chegutu. KRK66 was the best performing variety in terms of leaf yield at Chegutu in the 2016/17 season. For objective four, the tobacco variety effects were significant (p<0.05) on the S. gesnerioides germination and maximum germination distances. KRK66 and T74 had low S. gesnerioides seed germination percentages and maximum germination distances, whereas KM10, KRK22, KRK26, KRK28, KRK29, KRK60, KRK64, T70, T71, T72, T73, T75 and T76 had high S. gesnerioides seed germination percentages and maximum germination distances. It was concluded that the genetic diversity in S. gesnerioides was low. Trap cropping did not reduce S. gesnerioides seed banks. It was also concluded that bambara nut, common bean, sunn hemp, groundnut, cowpea and letting land lie fallow for two seasons reduced S. generioides infestation in tobacco grown in the third year at Mvuma. However, trap crops had no impact on S. gesnerioides emergence at Chegutu. The tobacco varieties produced similar leaf yields, which were below their potential at Mvuma. KRK66 was the best tobacco variety at Chegutu. It was estimated that S. gesnerioides could have contributed to the tobacco leaf yield loss of about 59.9 and 70 % in the 2016/17 and 2017/18 seasons, respectively at Mvuma. The tobacco leaf yield loss was 53.1 % at Chegutu. Based on the agar gel assay, it was concluded that KRK66 and T74 could have pre-attachment resistance to S. gesnerioides, whereas KM10, KRK22, KRK26, KRK28, KRK29, KRK60, KRK64, T70, T71, T72, T73, T75 and T76 were susceptible to S. gesnerioides infection.