In vitro phosphonate sensitivity of South African citrus Phytophthora nicotianae isolates

Sustainable citrus production in South Africa faces many biotic and abiotic challenges, including diseases caused by Phytophthora species.
The predominant Phytophthora species infecting citrus in South Africa is Phytophthora nicotianae. Phosphonates form an integral part of the management strategy against P. nicotianae. Their complex, multipronged mode of action makes them less prone to resistance development.
Despite this, Phytophthora isolates with reduced sensitivity to phosphonates have been reported, warranting an investigation into the phosphonate sensitivity of P. nicotianae isolates from South African citrus production regions.
Isolates of Phytophthora were collected from citrus-growing areas throughout the country and identified to species level by PCR-RFLP analysis and sequencing of the ITS region.
A representative sample of 50 isolates were screened for their in vitro sensitivity to potassium- and ammonium phosphite by an agar dilution method.
The EC₅₀ values were much lower than previously reported for P. nicotianae from citrus.
No isolate was found to have reduced sensitivity to either product, with no significant differences between products.
Surprisingly, isolates sourced from locations with a recent history of phosphonate use had EC₅₀ values significantly lower than those with no previous phosphonate exposure.
Despite the current findings, caution should still be exercised with the use of phosphonates to delay a shift towards reduced sensitivity in the P. nicotianae population in South Africa.
Resistance management strategies, such as the use of other oomycides in rotations or mixtures with phosphonates, are recommended.

XV International Citrus Congress

E. Theron, J.M. van Niekerk, J.E. van der Waals

ammonium phosphite, potassium phosphite, oomycide, root disease, resistance management

https://doi.org/10.17660/ActaHortic.2026.1448.69