Sorghum (Sorghum bicolor L. Moench) production in sub-Saharan Africa is seriously constrained by both biotic and abiotic stresses. Among the biotic stresses is witchweed (Striga spp.), a noxious parasitic weed causing major damage in cereal crops, such as sorghum. However, resistance through reduced germination stimulant production or altered germination stimulant composition provides a sustainable and most effective way for managing the parasitic weeds. Laboratory and glasshouse experiments were conducted using seven (7) sorghum genotypes to evaluate their resistance or tolerance the witch weed (Striga asiatica L. Kuntze). The first experiment was a laboratory agar gel assay arranged in a completely randomized design with six (6) replications to evaluate the effects of the seven (7) sorghum genotypes on the production of strigolactones by determining the percentage germination and the furthest germination distance of the Striga seeds. The second experiment was a seven (7) (sorghum genotypes)∗two (2) (Striga treatments) factorial glasshouse experiment conducted to evaluate the effects of Striga on sorghum growth, physiological and yield components of sorghum, Striga syndrome rating, and number of Striga per plant. The genotypes showed a significant (p<0.05) difference in germination percentage and furthest germination of Striga seeds in the agar gel assay. Genotypes SV4, Mahube, and ICSV 111 IN showed the least germination percentage and lowest germination distance, implying that these varieties either produced low strigolactones or altered their composition. In contrast, Kuyuma, Wahi, SV2, and Macia caused high Striga seed germinations and high furthest germination distances, suggesting that these sorghum genotypes were susceptible to Striga infection. The sorghum × Striga × time interactions were significant (p<0.05) on sorghum height. It was found that the heights of sorghum genotypes ICSV 111 IN and Mahube were not altered by Striga infection, but the heights of Kuyuma, Macia, SV2, SV4, and Wahi were reduced by Striga infection. The interaction of sorghum∗Striga effects was significant (p<0.05) on chlorophyll fluorescence. Striga infection did not alter the chlorophyll content of ICSV 111 IN and SV4. The sorghum∗Striga interaction effects were significant (p<0.05) on head index, leaf biomass, leaf index, root biomass, root index, plant biomass, and root : shoot ratio. Assessing Striga tolerance based on sorghum heights, chlorophyll content, and root : shoot ratio parameters, it could be concluded that the sorghum genotypes Mahube, ICSV 111 IN, and SV4 tolerated Striga infection, whereas Kuyuma and SV2 could be susceptible.

Late blight is caused by Phytopthora infestans (Mont.) de Bary which establishes quickly in Solanum esculentum L. (tomato); as a result, it makes the pathogen one of the most devastating plant diseases across the world. The control of late blight is difficult because P. infestans has advanced and complex enzymes and effecter molecules coded by avirulence genes. As such, a study was carried out at the University of Zimbabwe, Department of Plant Production Sciences and Technologies, between August 2018 and May 2019 to evaluate the efficacy of Moringa oleifera Lour. (moringa), Eucalyptus nigra R.T. Baker (gumtree), and Lantana camara L. (sensu lato) (lantana) extracts as biofungicides on late blight development on S. esculentum. The effect of acetone, ethyl acetate, and water extracts of M. oleifera, E. nigra, and L. camara on percent inhibition and radial growth were evaluated in the laboratory. In addition, the efficacy of different extract concentrations of M. oleifera, E. nigra, and L. camara L. on defence enzymes, disease incidence, disease severity, and yield parameters of S. esculentum were assessed in the in vivo experiment. Increasing concentration of the botanical extracts significantly ( p ≤ 0.05 ) reduced radial growth and increased percent inhibition of P. infestans. In addition, increasing botanical extract concentration significantly ( p ≤ 0.05 ) increased peroxidase (PОD), phenylalanine ammonia lyse (PAL), and polyphenol oxidase (PPO) activity. Disease incidence and severity were significantly ( p ≤ 0.05 ) reduced as the concentration of the botanical extracts increased. Similarly, marketable and total yield significantly ( p ≤ 0.05 ) increased with increase in the botanical extract concentration. Gas chromatography mass spectrometry (GCMS) revealed the presence of furfural, 5-methyl-4- (trifluoromethyl) pyrido, dodecanamide, cyclopentasiloxane, and decamethyl. From the research, it can be concluded that M. oleifera ethyl acetate, E. nigra ethyl acetate, and L. camara water extracts contain antifungal compounds and can be used as biofungicides in late blight management.

Weeds cause yield losses in desired crops through competition for sunlight, water, space, and nutrients. It is important to manage weeds in crop fields and aqua bodies using various management strategies. One of the most effective and efficient ways of managing weeds is through the use of herbicides. However, there is a need to understand the effects of these herbicides on the desired plants and/or the environment. Herbicide selectivity allows the application of herbicides in a field with both the desired crop and weeds. Herbicide selectivity is therefore described as, an application treatment at a given dosage that is toxic to some plant species but does not damage another species. However, many herbicides can be toxic at high dose rates even when applied to tolerant crops. There are several mechanisms of herbicide selectivity which are grouped into physiological and physical. Physical herbicide selectivity occurs when physical factors such as time, the position of application, plant morphological structure, and environment aid in selectivity. Physiological selectivity occurs when the plant species affect herbicide retention, penetration, movement, and detoxification. Herbicide selectivity is known to be a major cause of difficulties in controlling weeds. As a result, there is a need to understand how the environment, herbicides, and plants contribute to selectivity. This paper, therefore, provides insights into herbicide selectivity and how plants escape herbicide injury which enables species diversity.

Weed management through herbicides continues to be the most efficient and cost-effective component of integrated weed management (IWM) in crop production systems. However, the current production systems are characterised by intensive, inappropriate use of herbicides along with misapplication which creates selection pressure and leads to the rapid evolution of herbicide resistance in weeds. Currently, there are 511 unique cases of herbicide-resistant weed species globally, involving 266 species and these comprise 153 and 113 dicots and monocots respectively. To date, herbicide resistance has been reported from 96 crops in more than 71 countries. Furthermore, weeds have evolved resistance to 21 of the 31 known sites of herbicide action in different crops. Understanding the mechanisms of herbicide resistance is therefore essential which when coupled with the ecological and management factors that affect herbicide resistance would lead to the development of appropriate, profitable, and sustainable weed management strategies. The review is aimed at the development of herbicide resistance, mechanisms of herbicide resistance, factors that influence the rate of resistance development and management of herbicide resistance by growers.