Integrating Phosphonate Fungicide And Variety Resistance In The Management Of Potato Late Blight

Abstract

Potato is an important food and cash crop in sub-Saharan Africa highland regions. It production in terms of yields have stagnated for over decades due to several factors including diseases. Late blight caused by Phytophtliora infestans is one of the most devastating potato diseases in developing countries and its management has been very costly and difficult endavour for most poor farmers. The application of fungicides and development of resistance due to change in the population of pathogen has possed big challenges in controlling late blight hence the need for alternative management strategy. Studies were carried out in four sites in Kenya; to evaluate the resistance of potato varieties to late blight; to evaluate the potential of enhancing varietal resistance with phosphonate fungicide in management of late blight and to determine the appropriate application rate of phosphonate fungicide to optimize late blight control. Evaluation of the resistance of potato varieties was conducted in four sites; Kabete, Limuru, Njambini and Koibatek. Varieties Kenya Karibu and Kenya Mpya were rated as resistant. The most susceptible varieties were Arka and Desiree. Difference in the severity of the disease was observed between these sites. Phosphonate formulations were screened for their efficacy in controlling late blight at the University of Nairobi field station on variety Asante. The results showed that Phosphonates, which represent lower risks to human health and environment, were comparable to conventional fungicides on control and increasing the yields. Ridomil alternated with Mancozeb provided the best control but did not differ significantly with Agrifos 400. Fosphite, Phosguard, Potphos and Agriphite 600 also controlled late blight but at lower level. The highest yields were observed in plots treated with Ridomil alternated with Mancozeb followed by Agrifos 400 which had 45% yield increase while Fosphite had 38% yield increase. Two best Phosphonates formulations; Agrifos 400 and Fosphite were selected based on disease control and yield increase for use in with different potato varieties. Agrifos 400 and Fosphite were compared with Ridomil alternated with Mancozeb with different varieties in two locations; Kabete and Koibatek. Agrifos 400 was compared with Ridomil alternated with Mancozeb in two locations;

Njambini and Limuru. In all the four sites Agrifos 400 was not significantly different with Ridomil alternated with Mancozeb in control of foliar late blight in most of the varieties used and they were also comparable in yields with no significant differences. Agrifos 400 was applied at two different rates of 11Omll201 and 65mll201. High disease control was observed with Ridomil alternated with Mancozeb which did not differ with Agrifos 400 at the higher rate. Agrifos 400 at 65mll201 had significantly lower control compared to Agrifos 400 at higher rate. High yield were also observed with Agrifos 400 at 11Omll201 which was comparable to Ridomil alternated with Mancozeb. It is concluded that there exicist varieties with different resistant levels which should be exploited in breeding to improve potato genotypes at desirable levels. Results also show that phosphonates are clearly the effective disease control option for farmers given the potential health, environmental and economic benefits gained by using available phosphonate compounds. Higher yields and better disease control can be obtained by using Agrifos 400 at rate of 11Oml compared to the lower rate of 65ml per 201 of water. There is also the need to further investigate the timing and frequency of application of the phosphonates on the varieties with different resistance levels.