Distribution and management of groundnuts rust (Puccinia arachidis) and leaf spot (Cercospora arachidicola and cercosporidium personatum) diseases in Mtwara region

Dissertation

Groundnut productivity in Tanzania is low (909.2 kgha -1 ) compared to the global average (1 655 kgha -1 ). The low yield can partly be associated with groundnut rust, early leaf spot (ELS) and late leaf spot (LLS) diseases, all of which are the most common and serious diseases of groundnut in Mtwara region. The study aimed to establish occurrence of diseases and propose management options for increasing groundnut yield in the Mtwara region. A survey was conducted in two Agro-ecological Zones (AEZs) of Mtwara region, which are Coastal zone (CZ) and Eastern plateaux and mountain block zone (EPMBZ). Purposive sampling was used to select groundnut growers based on experience of at least five years of growing groundnuts in both zones. Occurrence of rust and leaf spot diseases was determined, whereby the disease incidence was established by counting the number of plants infected in the quadrat divided by the total number of plants per quadrat and multiplied by 100, while disease severity was calculated using a rating scale of infected plants per quadrat to determine the distribution of the diseases in each zone. Fungal isolates of ELS from CZ and EPMBZ were characterized based on their aggressiveness potential. A tetraplicated split-split experiment in a RCBD was implemented to determine the effects of tillage systems on development of groundnuts rust and leaf spot diseases in EPMBZ and CZ. Factor A was the AEZs, factor B was tillage systems which were conventional (0-15cm), deep (15-20) and zero (no till), and factor C was groundnut varieties (Mnanje-2009, Mangaka-2009, Naliendele-2009, and Pendo-1998). Disease incidence of early leaf spot was assessed at 30, 40 and 60 days after sowing (DAS) late leaf spot and leaf rust at 60 and 80 DAS by counting the number of plants infected and expressing it as a percentage of the total number of plants per quadrat. Rust and leaf spot diseases severity was assessed based on a rating scale of increasing severity of 1-9. Disease score 1 means 0% foliar infection; 2 for 1–5%; 3 for 6–10%; 4 for 11–20%, 5 for 21–30%; 6 for 31–40%; 7 for 41–60%, 8 for 61–80% and 9 for 81–100% of foliar area infection with plants having almost all leaves defoliated leaving bare stems. The survey revealed that there were significant differences (p < 0.05) between the two agro ecological zones in terms of groundnut productivity and farm sizes. Larger farm sizes were noted in EPMBZ the farmers had relatively large farm size by 32% compared to those in the CZ. Also the groundnut productivity in both cropping seasons 2017/18 and 2018/19 was reported higher in EPMBZ by 26% and 32% respectively compared to the groundnuts productivity in the CZ. The level of awareness to rust disease was higher (p < 0.05) in the CZ by 24% compared to that of farmers in the EPMBZ. There were highly significant (P<0.001) differences in incidences and severities of ELS, LLS and rust diseases between the two AEZs. The incidence and severity of ELS in EPMBZ exceeded those of CZ by 10% and 13% respectively. Similarly the incidence and severity of rust in EPMBZ exceeded the CZ by 17% and 11% respectively. Correspondingly the incidence and severity of LLS in CZ exceeded the same in EPMBZ by 22% and 14% respectively. Majority of farmers (p<0.001) in EPMBZ managed rust and leaf spot diseases by intercropping with legumes, usage of certified seeds, and crop residue management. Similarly majority 48% (p = 0.0014) in EPMBZ intercropped with tubers. On the other hand majority of farmers 29% (p<0.001) in the CZ managed the diseases by removal of volunteer plants and conventional tillage. The study found that there was significant difference (p<0.05) in isolates aggressiveness potential when inoculated in Pendo variety. The aggressiveness potential was measured in terms of number of infected plants per pot, infected branches per plant and the diameter size of lesion (mm). Similarly the ELS isolates were capable of reducing the height of inoculated plants, irrespective of their sources except one isolate. The study on the effect of AEZs, tillage systems and groundnut varieties one groundnuts yield and occurrences of rust and leaf spot diseases was established at the experimental plots located in the EPMBZ and CZ. Biomass productivity in the EPMBZ was significantly lower (p=0.005) with 1.075t/h which is less by 53% compared to the CZ. Mnanje-2009 groundnut variety yielded significantly higher (p = 0.003) in pod and (p<.001) in biomass weight as it outperformed Naliendele-2009, Pendo-1998 and Mangaka-2009 varieties by 32%, 14% and 16% respectively. The studies of rust and leaf spot diseases occurrence propose that variety Mnanje-2009 should be prioritized in all zones due to its inherent resistance level to rust and leaf spot diseases and high yielding. Pendo-1998 variety being a short maturity variety can be grown earlier at the commencement of rain to escape the polycyclic nature of rust and leaf spot pathogens. Late sowing faces a challenge of secondary inoculum production and reinfection under field conditions. The study recommend the use of higher yielding and resistant varieties like Mnanje-2009 in coupled with conventional tillage, removal of volunteer plants, managements of crop residues and intercropping with tubers, cereals and legumes these practices would minimize the indiscriminate use of fungicides to the environment and lessen rust and leaf spot diseases to resource poor farmers who cannot achieve the costs of fungicides.

Ministry of Agriculture