Cultural studies and assessment of seed borne inoculum of Pyricularia grisea (Anamorph Magnaporthe Grisea) on and in Finger Millet Seed and its implication on blast incidence and severity

Abstract

Blast of finger millet caused by Pyricularia grisea anamorph Magnaporthe grisea has been reported to be a major constraint of finger millet (Eleusine coracana) production in Kenya causing 10 - 90 % losses in farmers fields. During a study on finger millet blast conducted between December 1995 and February 1997 various laboratory and field experiments were conducted to study finger millet blast disease and address the following objectives: To establish the seedbome nature of Pyricularia grisea anamorph Magnaporthe grisea in and on finger millet seeds; to establish the best cultural conditions of the pathogen, Pyricularia grisea anamorph Magnaporthe grisea; to determine the efficiency of the various seed assessment techniques for evaluation of seedbome inocula of Pyricularia grisea anamorph Magnaporthe grisea; to determine the relationship between seedbome inocula of Pyricularia grisea and disease incidence in the subsequent finger millet crop; to determine the relationship between time of infection of plant with Pyricularia grisea anamorph Magnaporthe grisea and level of disease severity and level of contamination of harvested seed. The methodology used included for seed assessment of farmer seeds sampled from Kisii, Bungoma and Busia/Teso were blotter, rolled paper towel and agar plate test; for cultural studies five different media, 4 different pH levels, 3 isolates and 4 different lighting regimes were used; for artificial inoculation 5 different inoculation stages were used; for planting out seeds with different levels of P. grisea 6 different levels were used .. P. grisea anamorph Magnaporthe grisea was found to sporulate best on oatmeal agar (OMA) at pH 7 and a 12 hr lighting / 12 hr darkness regime and hyphal growth was best on media FMLEA(finger millet leaf extract agar)at pH 8 and 24 hours lighting regime. P. grisea anamorph Magnaporthe grisea also exhibited concentric zonation and sectoring on finger millet seed extract agar (FMSEA) and OMA. Three P. grisea anamorph Magnaporthe grisea isolates were obtained from infected seeds obtained from three districts of Busia, Bungoma and Kisii. The Kisii isolate was found to produce most spores (p ~ 0.05) on media followed by isolates from Bungoma and Busia respectively. The isolate from Busia had the fastest mycelial growth rate in mm/day on media followed by the isolates from Kisii and Bungoma respectively. Thirty four seed samples of Finger millet collected from farmers in Busia/Teso, Kisii and Bungoma and 7 samples provided by ICRISA T showed infection of P. grisea anamorph Magnaporthe grisea ranging from 0 - 14 % when assay method was blotter test. Infected seed samples showed poor germination « 80% germination) which is below the 80 % germination requirement. Out of these farmers seeds only one would qualify for a quality certificate in Kenya. Grey/black discoloured seeds had more infection of P. grisea and seed infection counts were higher in the blotter than in the rolled paper or agar test methods, showing that the blotter test would be a more appropriate seed assessment test method than the other two. Epidemics of finger millet blast caused by P. grisea anamorph Magnaporthe grisea were initiated at 5 different plant growth stages i.e. 1, 2, 3, 4 & 5 corresponding to stem elongation, inflorescence emergence, anthesis, milk development and ripening, on a finger millet genotype KNE 479 during the 1996 long rains at Kiboko and Kabete and 1996/97 short rains at Kabete. Stem elongation stage was found to be most susceptible (p ~ 0.05) when various disease index parameters were considered, while milk development stage had most (p ~ 0.05) contamination of P. grisea anamorph Magnaporthe grisea on harvested seed. In stem elongation stage, yield losses as great as 11 % were recorded from inoculated plots when seed yields were compared to those of control plots (uninoculated). The 1000 seed weight was significantly reduced in some inoculated treatments. The high yield losses, especially in 1000 seed weight were indicating that P. grisea anamorph Magnaporthe grisea could be a greater problem in seed production fields. Percent yield reductions were positively correlated (p ~ 0.05) to panicle, neck and leaf blast. A disease scoring system for leaf, neck and panicle blast was designed for the experiment especially for use in the field. Infected seed samples of 0, 2.5, 4.25, 6.00, 7.75 and 14.00 percent incidences of P. grisea anamorph Magnaporthe grisea were grown in the field, exhibited less than 2 % severity on the leaves and no panicle or neck blast was recorded. Final incidences in relation to leaf blast varied from 26 to 38 %. In recommending, P. grisea anamorph Magnaporthe grisea would sporulate best under media OMA at pH 7 and lighting regime of 12 hours lighting and 12 hours darkness. Control measures in the field for blast should start before the stem elongation stage. Farmers should be trained on production of healthy seeds, since they do use the seed from the harvested yield.