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Witch weed (Striga gesnerioides) is a root parasitic weed of tobacco and cowpea in Sub- Saharan Africa. It is very difficult to control and host plant resistance is the most promising effective method affordable to small-scale farmers. This study was done to screen local tobacco genotypes for tolerance and resistance to Striga gesnerioides infection. Laboratory screening experiments were conducted during the 2016/17 season using fifteen local tobacco genotypes. The experiments were done using an Agar gel technique at the University of Zimbabwe, with treatments replicated four times in a Randomised Complete Block Design (RCBD). All tobacco genotypes significantly (p

Striga infestation presents a major constraint to crop production in agricultural systems. The tobacco witch weed, Strigagesnerioides occurs widely in Africa and other parts of the world where it is known to parasitize mainly cowpea (Vignaunguiculata) and other dicotyledonous plants. In Zimbabwe, the parasitic weed was first confirmed in the Nyikavanhu area of Mvuma in the year 2007 and research was initiated to study this parasitic weed and recommend control strategies. A few more reports have been received of the parasitic weed infesting tobacco in other growing regions. However, despite having been reported widely on cowpea in other countries, S. gesnerioides has not been documented on this and related crops in Zimbabwe. Interestingly, in previous research conducted in Zimbabwe, it was established that cowpea induces suicidal germination of S. gesnerioides seeds but resulting in no further developmental stages such as haustorial attachment. This led to the conclusion that the species strain reported elsewhere on cowpea could be a different biotype from the one currently parasitizing tobacco in Zimbabwe. Thus it was felt that cowpea could then be used in tobacco crop rotations as a trap crop by inducing suicidal germination of parasite’s seed. This current work reports the occurrence of S. gesnerioides on other plants in addition to tobacco.

Ozone (O3) is the most important gaseous air pollutant in the world because of its adverse effects on vegetation in general and crop plants in particular. Since nitrogen dioxide (NO2) is a precursor of ozone, studying the implication of sequences of these two gases is very important. Hence, the effects of sequences of fluctuating levels of ozone (O3) and/or nitrogen dioxide (NO2) on growth and stomatal diffusive resistance were studied in Radish (Raphanus sativus L.). Nitrogen dioxide was applied either alone early in the day (06:00 to 10:00 hr), late (18:00 to 22:00 hr), both early and late (06:00 to 10:00 hr and 18:00 to 22:00 hr) or early with ozone (O3 from 10:00 to 18:00 hr), late with ozone, and both early and late with ozone. Ozone was also applied alone (10:00 to 18:00 hr), while control plants (C) were not exposed to either gas. The exposure profiles for both gases approximated sine waves with peak concentrations of 120 parts per billion by volume; nl l-1. In the case of O3, this is close tothe reported threshold for adverse effects, while with NO2 it is below the reported threshold. Ozone alone had no effect on growth after 21 days; while NO2 caused a significant increase in growth only when applied alone early. Exposures to NO2 in sequence with O3 had negative effects on growth. In the control treatment, stomata tended to be relatively closed in the morning and late afternoon. Nitrogen dioxide alone enhanced this closure, while O3 alone or with NO2 countered the closure.

Ozone (O3) is the most important of the phytotoxic gaseous air pollutants. It causes substantial decreases in crop yields worldwide and bears adverse effects on vegetation in general. On the other hand, nitrogen dioxide (NO2) is the air pollutant most likely to be associated with O3 because it is a precursor of O3. The objective of this study was to determine the effects of sequential exposures to nitrogen dioxide (NO2) and ozone (O3) on chlorophyll fluorescence in radish, Raphanus sativus L. Radish plants were exposed daily to O3 or NO2, or sequences of the two gases. The exposure profiles for both gases approximated sine waves with peak concentrations of 120 ppb (parts per billion by volume, nl l-1). In the case of O3, this is close to the reported threshold for adverse effects; while for NO2 it is below the reported threshold. The sequences involved different combinations of exposures to NO2 from 06:00 to 10:00h and/or 18:00 to 22:00hr and O3 from 10:00 to 18:00hr. Relative to the control, early and early + late NO2 resulted in stimulations of quantum yield (Y) and photochemical quenching (qP), with late NO2 resulting in little or no change. In contrast, early, late and early + late NO2 in combination with O3 resulted in progressive reductions in these variables. The overall effect of O3 treatment was to stimulate quantum yield and qP, both of which are indicative of increased photochemistry. Late NO2 exposures caused no significant effects relative to the control. However, late NO2 failed to result in a significant stimulation of photochemistry in the chloroplast, but caused significant residual increases in non-photochemical quenching (qN) during the middle of the day, responses which imply increased photo-protection capacity. Stimulation of quantum yield and photochemical quenching by O3, O3 + late NO2, O3 + early NO2, early NO2 and late + early NO2 implies thatCO2-fixation was limited by processes at PSII. However, apart from the early NO2 related stimulation of qN, all exposures involving O3 led to a decrease in qN implying inability to regulate photosnynthesis resulting from changes in the thylakoid membrane. In the case of NPQ, all exposures including O3, decreased this parameter suggesting impaired proper functioning of the xanthophyll cycle associated with the light-harvesting complex of photosystem II.

Sponsored by UNIDO, Zimbabwe in 2004, resolved to train about 28,033 tobacco farmers in five years so as to phase out the last 182 tonnes of methyl bromide then used in tobacco seedling production. The replacement technology chosen was the floating tray system. Floating tray system equipment for effective phase out was distributed to farmers with the overall objective of phasing out methyl bromide by 2009. Largely, most of the farmers were trained and equipment distributed for the effective phase- out as planned. This survey, carried out seven years later in 2011, intended to evaluate the adoption of the floating tray system by farmers. This information is required in order to assess the country’s readiness for the 2015 banning of methyl bromide use in tobacco seedbeds. In addition, the survey sought to find out the role tobacco production played in deforestation and food security among other minor objectives. The survey utilized both qualitative and quantitative methods and was administered mainly on a face-to-face manner at the Boka and Tobacco Sales auction floors in Harare during the 2010/2011 selling season. In essence, the survey found out that the majority of the growers (44%) were males over 36 years old and that 76% were smallholder farmers farming on land below two hectares in size. Only 11% used the floating tray system although 61% had heard about the technology and could not use it citing the cost (48%), difficulties in procuring the substrate used in the technique, and lack of know how (39%) among other reasons. An alarming 74 % sterilised their seedbed by burning wood. Almost all farmers (91%) used the proceeds from the tobacco crop to support their maize crop. The few who used the floating tray system agreed that, it was easy (96%) and that an on-farm substrate would be better. In addition, they agreed that procurement of substrate was a problem (64%) and almost all (91%) wished to be trained in both the making of on-farm substrate from farm waste, and in the use of the floating tray system. Almost all used chemicals as a risk aversion technique. We concluded that, the floating tray system would be a recluse of a few after phase-out unless supportive measures are put in place. We also found that the use of indigenous wood in the production of tobacco would probably lead to deforestation.Key Words: Deforestation, maize, ozone layer, tobacco