Investigations on the causes and effects of peanut (Arachis hypogaea L.) seed discoloration [Philippines].

A black discoloration had been observed in peanut seeds in the field and in commercial warehouses. Seven fungi were isolated from the black discoloration in peanut cotyledons. These were Aspergillus flavus Link, Aspergillus Tamarii Kita, Aspergillus niger van Tieghem, Aspergillus chevalieri Thom and Church, Mucor sp. Michell ex Fr., Macrophomina phaseolina (Tassi) Goid., and sterile white mycelia. M. phaseolina was consistently isolated from the black discolored cotyledons. In addition, A. flavus, A. tamarii or A. niger were observed growing from the black cotyledons when the plates were further incubated for 48 h. Pathogenicity tests showed that these fungi produced different forms or shades of discoloration depending on the color of each fungus. The black discoloration in the cotyledons which was similar to the one observed in the field was produced by M. phaseolina. The discoloration produced by the other fungi is also described. M. phaseolina developed in peanut seeds and produced the black symptom in the cotyledons at relative humidities equivalent to 95%, 87%, 80 % and 75 %. The development of symptom was fastest at 95% and slowest at 75% RH. Peanut varieties UPL-Pn4, UPL-Pn6, BPI-P9 and CES 101, developed the black symptom in the cotyledons when inoculated with M. phaseolina. Among the four varieties, UPL-Pn4 had the highest percentage of discoloration whereas CES 101 had the lowest percentage of discoloration. Peanut samples bought from commercial warehouse in Isabela [Philippines] province were classified into apparently healthy, sound seeds and discolored seeds. The aflatoxin level of the healthy looking seeds were always lower than the aflatoxin level of discolored seeds. This does not mean however, that the aflatoxin content of healthy looking peanuts is always within the limit of aflatoxin level set by the Food and Drug Administration (FDA) because it was shown that healthy looking peanuts may contain aflatoxin levels significantly higher than 30 ppb, the limit set by FDA.