Response Of Avocado (Persea Americana Mill.) Rootstocks To Soil Flooding And Inoculation With Phytophthora Cinnamomi

ABSTRACT

Avocado plants (Persea americana Mill.) belongs to the family lauraceae. Avocado fruit has high amounts of fats and proteins, high dietary fibre, vitamins and potassium. It is known to be the most nutritious of all the fruits. The most devastating pathogen of avocado plants is a fungal species, Phytophthora cinnamomi which cause avocado root rot. The fungus is the most widely distributed of Phytophthora species. It affects growth and physiology of plants and even leading to death of plants. When this pathogen is combined with flooding, the potential for asphyxiation leading to root decay and death is increased. Even though several studies have evaluated the tolerance of different avocado rootstocks for tolerance to Phytophthora cinnamomi there are few such studies in Kenya which have characterized the pathogen and evaluated the effect of the pathogen on the physiology of avocado rootstocks under flooding conditions. The objectives of this study therefore were to isolate and morphologically characterize P. cinnamomi, and investigate growth and physiological responses of avocado rootstocks to P. cinnamomi under water logging conditions. Four avocado rootstocks (Puebla, Fuerte, Booth7 and Pinkerton) were obtained from orchards in Maseno, Nyando, Kisumu East and Busia-Budalangi of Western Kenya. The pathogen was isolated from the soil through plating on selective medium and by baiting methods. Ripe avocado fruits were used as baits. Morphological characterization of the pathogen was carried out. Avocado rootstocks were planted in 10 litre plastic pots containing sand - soil mixture of ratio 1:2 for 3 months and then inoculated with P. cinnamomi under flooded and non-flooded conditions. The plastic pots grown with plants were laid out in a greenhouse in a completely randomized design. Growth was assessed by determining the plant heights, stem diameter, plant fresh weights, plant dry weights, and leaf area. The seedlings were uprooted at the end of the study for root necrosis examination. Data on chlorophyll concentration and chlorophyll fluorescence parameters were collected using a spectrophotometer and portable fluorescence monitoring system respectively. The data collected was subjected to analysis of variance. Treatment means were separated and compared using least significant difference at 0.05. Flooding and inoculation with P. cinnamomi significantly (p≤0.05) reduced the growth of the avocado rootstocks. This was reflected in the reduction of plant height, stem diameter, leaf area and dry weights. Disease incidence led to reduced plant growth and massive death of avocado rootstocks such as in Fuerte and Pinkerton under flooded and inoculated conditions. Significant reductions chlorophyll a, b and total chlorophyll concentration were also evidenced from this study. Generally Fv/Fm, ΦPSII and ETR of the four avocado rootstocks decreased under flooding and P. cinnamomi inoculation treatments, an indication that photosynthetic apparatus were affected contributing to the reduction in avocado growth. A significant interaction was found between the various treatments and avocado rootstocks as far as plant height, plant stem diameter, plant fresh and dry weights, disease incidence, chlorophyll a, b, total chlorophyll concentration, and chlorophyll fluorescence parameters. Out of the four selected rootstocks studied, Puebla rootstocks responded better to flooding and P. cinnamomi inoculation, and therefore may be recommended for growing in flood affected regions of Kenya where P. cinnamomi infestation is also a common problem.