IN-VITRO EVALUATION OF FUNGICIDAL EFFECT OF NEEM SEED OIL AGAINST Alternaria porri (Ellis) Cif., CAUSING PURPLE BLOTCH DISEASE OF ONION

Purple blotch of onion, caused by Alternaria porri is a serious foliar disease of onion in Nigeria
that reduces yield and bulb quality. Neem seed oil contains a complex mixture of compounds
with well-documented insecticidal and antimicrobial properties. This research assessed the in
vitro fungicidal activity of neem seed oil against Alternaria porri using the poisoned-food
technique. Neem seed oil was tested at 3 ml (T1), 5 ml (T2), 7 ml (T3), and 10 ml (T4) in potato
dextrose agar (PDA). Completely Randomized Design (CRD) was used and the treatments were
repeated six times. Radial mycelial growth was measured and percent inhibition relative to
control was calculated. Data were subjected to Analysis of Variance (ANOVA) and significant
means were separated using Least Significant Difference at P ≤ 0.05. Results revealed that all
the levels of neem seed oil used inhibited the mycelial growth of A. porri but with significant
variation. There was no significant difference among the treatments 48 hours after inoculation.
The mean colony diameter of treatment T1 and T2 (1.67 cm and 1.65 cm), respectively was
statistically similar with the control (1.70 cm). After 96 and 120 hours of inoculation, there was
significant (P ≤ 0.05) difference among the treatments. Treatment T4 (10 ml) significantly
reduced growth and sporulation of A. porri and had lowest mean colony diameter and highest
percent growth inhibition (2.06 cm and 43.51%), respectively. Treatment T4 (10 ml)
significantly reduced growth and sporulation of A. porri throughout the experiment. Complete
inhibition was not observed at high volume. These findings revealed that neem seed oil has
strong in-vitro antifungal activity against A. porri that caused onion purple blotch disease.
Further research that will evaluate neem seed oil as a component of integrated management for
purple blotch disease in Nigerian onion production under screen-house and field conditions is
recommended.