6 Records out of 22207 Records

Adapting spraying regimes to two late blight resistant potato varieties and testing for metalaxyl resistance in phytophthora infestans

Author: Ngatia, Grace Wanjiru

Awarding University: Jomo Kenyatta University of Agriculture and Technology, Kenya

Level : MSc

Year: 2008

Holding Libraries: Jomo Kenyatta University of Agriculture and Technology Library ;

Subject Terms: Pesticides ; Crop diseases ; Blight ; Phytophthora infestans ; Solanum tuberosum ;


Nine fungicide spray regimes were tested to complement the resistance of Asante (CIP 381381.20) and Tigoni (CIP 381381.13) potato varieties against late blight over two seasons at the KARl seed multiplication centre in Njabini (Latitude: 0? -44' 60 N, Longitude: 36? 40' 0 E). The nine treatments were based on different spray intervals and/or combinations of a systemic (Ridomil Gold) and contact (Dithane M45) fungicide. All the treatments reduced disease severity, increased yield and marginal net benefit substantially when compared to the unsprayed control treatment. The damage threshold treatment where Ridomil was sprayed at the onset of disease symptoms and then at 5, 10 and 15% disease severity, gave the best control of late blight for both varieties with a disease severity ranging from 12.7 to 15.2%. This treatment also gave the highest net benefit in season 2 of>Ksh.80,000 per hectare for both varieties. However, it also required the highest investment of Ksh. 9,880 per hectare. The one spray treatment, where Ridomil was sprayed only at the onset of symptoms, required the least investment of Ksh.l 026 and 1625 per hectare for season 1 and 2 respectively and gave the highest marginal rate of return which ranged from 1678 to 6794% for both varieties over both seasons. The two spray treatment, where Dithane M45 was sprayed at emergence and Ridomil sprayed at the onset of symptoms effectively complemented the host resistance of both varieties, as it satisfactorily suppressed the disease and gave additional marginal rates of return to The one spray treatment, ranging from 123 to 3164% with a minimal addition investment of less than Ksh.1 000 per hectare. The two spray treatment was found to be cost effective and therefore recommended to complement the resistance of Asante and Tigoni varieties. Potato leaf samples infected with P. infestans were collected in potato production zones of Meru and Nyandarua districts and isolated on 10% V8 agar supplemented with antimicrobial compounds. The sensitivity of isolates to metalaxyl was assessed by measuring the radial growth of the colony on 10% V8 agar amended with 0, 5 or l Ouug/ml of metalaxyl. Overall, 6% of the isolates were resistant to metalaxyl, 6% were intermediate and 88% were sensitive. The prevalence of isolates that grew in the presence of metalaxyl with growth of <40% of the control at 5 ug/ml was higher in the isolates obtained from the Nyandarua area than those from Meru. In addition, the LB2004 version of LATEBLIOHT, a mathematical model that simulates the effect of weather, host growth and resistance and fungicide use on the asexual development of P. infestans on potato foliage was used to simulate late blight disease for two potato varieties, Asante and Tigoni. Model predictions using general category parameters overestimated disease, perhaps due to inaccurate estimates of the fitness parameters. The fitness parameters were then calibrated using data collected from Njabini. The new parameters determined through calibration for Asante variety were: sporulation rate (SR) of 1.2 x 108 sporangia m-2 day' I , lesion growth rate (LOR) of 4 x 10-3 m day' I , latent period (LP) of 4.2 days and infection efficiency (IE) of 0.8 while for Tigoni variety the LOR was found to be 3.9 x 10-3 m day' I , SR was 1.175 x 108 sporangia m-2 day' I , LP was 4.2 days and IE was 0.8. When calibrated parameters for Asante and Tigoni varieties were used for simulation, model predictions fit the observed data.

Dynamics of potato late blight on tubers and farmers' management of the disease in Kenya highlands.

Author: Nyankanga, Richard Ombui

Awarding University: Cornell University, USA

Level : PhD

Year: 2003

Holding Libraries: University Microfilms International ;

Subject Terms: Crop diseases ; Farming ; Phytophthora infestans ; Pesticides ;


The focus of this dissertation was on understanding the dynamics of field infection of potato tuber infection by Phytophthora infestans. It also examines methods that could be used to manage tuber blight in the field. Factors affecting the relationship between foliage late blight and tuber infection were studied both in New York and in Kenya. The relationship between levels of foliage blight and tuber infection was examined by varying disease on foliage by different fungicide regimes. Fungicide application either in terms of different rates or spray intervals did not have a significant effect on tuber blight incidence. No significant association was found between foliage disease levels and tuber infection. Generally, cultivar characteristics, soil temperature, and precipitation events moderated the relationship between foliage and tuber blight. Development of regression models using tuber resistance and depth, soil temperature, and precipitation had moderate predictive ability in ny (r<super>2</super> = 0.56) but poor prediction in Kenya (r<super>2</super> = 0.34). Evaluation of hill sizes and mulch as barriers or filters of inoculum before reaching the soil was assessed in ny. Hilling suppressed tuber infection moderately under moderate epidemics but was ineffective under high disease pressure. Mulching was ineffective under the epidemics in the two years the experiments were conducted. Potato cultivars and advanced clones were assayed in vitro over two years in Kenya for tolerance to tuber blight. Relative resistances of the genotypes differed significantly among cultivars and advanced clones. Resistance in tubers was marked by high variability and may be due to minor genes. No significant correlation was observed between components of tuber resistance and foliage blight ratings indicating that tuber resistance might be different from the resistance in the foliage. A range of tuber blight resistance was observed from which lines with resistance may be developed. A survey of farmers was conducted in three potato growing areas in Kenya in 2000 and 2001 to evaluate farmers' perception, knowledge and management potato late blight. Results indicated that farmers had a limited understanding of late blight. High cost of fungicide, poor application techniques, incorrect dosage of fungicides, and preference of susceptible cultivars were among the reasons contributing to lack of adequate control of late blight. Farmers' preference for potato cultivars was not based on late blight resistance. Other constraints like lack of markets, prevalence of bacterial wilt and scarcity of quality seed were identified as serious constraints of potato production. Improvement of potato late blight control should include provision of knowledge and addressing these other constraints.

Epidemiology and control of late blight (Phytophthora infestans) on tomato (Lycopersicon esculentum) : a study in small-holder farms in Kenya

Author: Karanja, Daniel Kimani

Awarding University: Moi University, Kenya

Level : MPhil

Year: 2001

Holding Libraries: Moi University Margaret Thatcher Library ;

Subject Terms: Tomatoes/Blight/Phytophthora infestans/Lycopersicon esculentum/Pesticides/Fungi/ ;


Late blight caused by the fungus Phytophthora infestans (Mont.) de Bary is one of the most important limiting factors in the production of tomatoes (Lycopersicon esculentum Mill.), particularly in Kenya. Therefore, there is a need to develop appropriate control strategies. During the 1998/99 short rains growing season, on farm field trials and laboratory experiments were conducted in two contrasting agroecological zones, to determine the temporal and spatial progress of the disease and possible conventional and non-conventional strategies for the control of late blight. During the experimental period, repeated assessments of the disease over time enabled consideration of both the spatial and temporal dynamics of late blight epidemics in the field. Analysis of spatial pattern of the disease showed that the disease epidemic was initiated from discrete and regularly distributed foci of tomato plants but did not provide enough evidence for the presence of infected planting material. Subsequent spread of infection led to an aggregated spatial pattern of infected plants, which became regular as the entire crop became infected. An examination of disease progress curves showed the possible influence of meteorological factors on the temporal progress of late blight on tomato. Although there was low incidence and severity of the disease at a lower altitude rather than higher agro-ecological zone, during initial stages of disease development, there was not enough evidence to correlate the influence of environmental factors on disease development. This was due to the occurrence of an unusually dry spell during the growmg season. Preliminary investigations showed that foliar applications of Dithane? M-45, Milk and Ridomil? MZ 63.5 WP reduced the severity of late blight on the susceptible tomato cultivar, Money maker compared with the unsprayed crop and resulted in significant increase in yield. Tomato cultivars Cal-J, Arletta and Roma VF had low levels of disease severity compared with Money maker but there was no significant yield difference. The implications and prospects for these control measures in relation to cost, development of resistance to the pathogen and environmental effects is discussed.

Effects of fungicide dosages, staking and plant densities on development of late blight on tomatoes in Tigoni.

Author: Kinyagia, Benson M

Awarding University: Kenyatta University, Kenya

Level : MSc

Year: 1998

Holding Libraries: Kenyatta University Moi Library ;

Subject Terms: Lycopersicon esculentum/Crop diseases/Pesticides/Phytophthora infestans/Lycopersicon esculentum/Tigoni, Kenya/ ;


Tests were conducted in the field to investigate the effects of (i) fungicide, dosage and frequency of application, (ii) staking and (iii) different plant densities on development of late blight disease (Phytophthora infestans) on tomato (Lycopersicon esculentum). All the tests were replicated four times and arranged in a randomized complete block design. In order to investigate effects of fungicide dosage on disease development, tomato plants were sprayed with metalaxyl-mancozeb (Rodomil-MZ) at 100% dosage (160 gm/20 l. of water), 75% (120 gm/20 l. of water) dosage, 50% (80gm/ 20 i. of water) and non-fungicide control after every 7 day intervals. To investigate the effects of reduced frequency of fungicide application on the disease, spraying was done after 7, 14 and 21-day intervals. To investigate the effects of staking on late blight plants were staked using 2-m long sticks. To determine the effects of different planting densities on late blight disease development, tomato plants were planted with spacing of 120 cm by 80 cm, 90 cm by 60 cm and 60 cm by 40 cm. Plant performance assessment was based on plant height, fruit weight, number of fruits, and number of culls and expressed in tonnes per hectare. Fungicide dosages influenced disease development differently. The 100% dosage had significantly (p=0.05) the lowest mean disease severity index (MDSI) of between 1.25 and 2.63 and mean disease incidence index (MDII) of between 2.13 and 5.13, while the non-fungicide application (control) had the highest MDSI of between 4.25 and 5.5 and MDII of between 5.5 and 6. However, those sprayed at 100% and 75% did not differ significantly (p=0.05). There was a significant difference in MDSI and MDII when spraying was done after 7, 14 and 21-day intervals. The 21-day interval treatment had significantly the highest MDSI of between 3 and 4.75 and MDII of between 4.25 and 5.63. The MDSI of between 1.75 and 3.38 and MDII of between 2.38 and 4.63 associated with spraying after 7 and 14 days during the long rains were found to be significantly (p=0.05) different. Staking plus fungicide gave MDSI of between 1.25 and 2.38 and MDII of between 1.25 and 4.55 that were significantly (p=0.05) lower than those associated with fungicide alone, staking alone, and no-fungicide no-staking treatments (control). Significant (p=0.05) differences in MDSI and MDII were detected among various plant spacing. The lowest MDSI of between 1.13 and 3.5 and MDII of between 1.25 and 5.5 were associated with 120 cm by 90 cm spacing. The highest MDSI of between 3.25 and 5.38 and MDII of between 4.13 and 6 were associated with 60 cm by 40 cm spacing. In conclusion it can be noted that fungicide dosage and frequency of application are critical in management of late blight disease. Staking plus fungicide treatment was found to be highly effective in control of the disease, however, staking alone not a sufficient measure of control. Plant densities were to have notable influence on development of late blight disease.

Screening and selection of potato clones and varieties for resistance to late blight phytophthora infestans (mont.) de Bary

Author: Wabule, Maey N

Awarding University: University of Nairobi, Kenya

Level : MSc

Year: 1981

Holding Libraries: University of Nairobi Upper Kabete Library ;

Subject Terms: Crops/Genetics/Crop diseases/Blight/Phytophthora infestans/ ;



Use of electrophoresis and activity of peroxidases in selection of potato (Solanum tuberosum) clones resistance to late blight (Phytophthora infestans)

Author: Nyachae, Francis O

Awarding University: University of Nairobi, Kenya

Level : MSc

Year: 1979

Holding Libraries: University of Nairobi Upper Kabete Library ;

Subject Terms: Crops ; Solanum tuberosum ; Biochemistry ; Crop diseases ; Blight ; Fungus ; Phytophthora infestans ;