Hardpan and maize root distribution under conservation and conventional tillage in Agro-Ecological Zone IIA, Zambia

Hardpans (plough/hoe pans) are commonly believed to restrict plant root growth and crop yields under conventional small-scale agriculture in sub-Saharan Africa. This study questions the notion of widespread hardpans in Zambia and their remedy under conservation tillage. Soil penetration resistance was measured in 8x12 grids, covering 80 cm wide and 60 cm deep profiles in 32 soil pits. Large and fine maize roots were counted in 8x6 grids. Soil samples from mid-rows were analysed for pH, exchangeable H+, exchangeable Al3+, cation exchange capacity, total N and extractable P (Bray 1) at six depths from 0-10 to 50-60 cm. Cultivation-induced hardpans were not detected. Soils under conservation tillage were more compact at 5 cm depth than soils under conventional tillage. No differences in root distributions between conservation and conventional tillage were found. Maize ( Zea mays L. ) roots were largely confined to a relatively small soil volume of about 30 cm x 30 cm x 30 cm. Root growth appeared to be restricted by a combination of low concentrations of N and P. Soil acidity and Al saturation appeared to play a minor role in root distribution. L-shaped taproots in soils under manual tillage reported earlier were not necessarily due to hardpans, but may rather be caused by temporarily dry, impenetrable subsoils early in the rain season. There is no scientific basis for the recommendation given to farmers by agricultural extension workers to “break the hardpan” in fields under manual or animal tillage in the study areas.

FIELD TOLERANCE OF SELECTED VARIETIES TO AND FUNGICIDE EFFICACY AGAINST ALTERNARIA BLIGHT OF SWEET POTATO

Alternaria blight (AB) of sweet potato ( Ipomoea batatas L. ), caused by Alternaria spp., was recently reported in South Africa, but is common in southern and eastern Africa. Elsewhere in the world, AB is controlled primarily using resistant varieties. Twenty-five sweet potato varieties/breeding lines, from different origins were assessed for tolerance to AB. The materials were planted in fields having a history of AB disease and rated for tolerance based on a General Disease Index (GDI), with the lowest scores representing tolerance, and the higher scores representing susceptibility. Variety 199062-1 had the lowest GDI value, and was the most tolerant to AB; while W119 had the highest GDI value and was the most susceptible to the disease. Other varieties/breeding lines showed a variation in GDI values between most tolerant and most susceptible. Among the fungicides tested under field conditions, the mixture azoxystrobin-difenoconazole was the most effective in reducing AB intensity. Fungicides pyraclostrobin-boscalid, unizeb, azoxystrobin-chlorothalonil and cymoxanil-mancozeb were also effective against the disease.

In vivo antifungal activity of neem oil and aqueous extracts against leaf spot disease caused by Cercospora abelmoschii on okra

The cercospora leaf spot, caused by Cercospora abelmoschi Ellis and Everhart, is quite common in okra culture. Therefore, this study aimed to evaluate the efficiency of aqueous extracts of neem ( Azadirachta indica A. Juss), citronella ( Cymbopogon nardus (L.) Rendle), eucalyptus ( Eucalyptus grandis L.), ecolife®, A. indica oil and fungicide cercobin 700 PM® in control of cercospora leaf spot on okra in greenhouse. The extracts and neem oil were tested in concentration 10%, the fungicide cercobin 700PM® in dose 2.5 g.l-1, applied 10 days after pathogen inoculation by leaf spray and the citric biomass extract ecolife® in concentration 5.0 ml.l-1, applied 10 days before pathogen inoculation. All treatments, except ecolife®, were effective in controlling cercospora leaf spot and may be recommended as alternatives in agroecological systems.