IMPACT OF PLANT GROWTH PROMOTING RHIZOBACTERIA AS BIO-CONTROL AGENTS ON CITRUS NEMATODE, TYLENCHULUS SEMIPENETRANS INFECTING BALADY ORANGE (CITRUS SINENSIS L.) AND IMPROVING ITS PRODUCTIVITY

Abstract

Three bacterial species of plant growth promoting rhizobacteria (PGPR) namely Paenibacillus
polymyxa, Methylobacterium mesophilicum and Methylobacterium radiotolerans were tested alone or combined
with humic acid as bio-control agents against the citrus nematode Tylenchulus semipenetrans under laboratory
and field conditions. Results cleared that; all tested PGPR species produced IAA, HCN, ammonia, chitinase and
protease enzymes and also solubilized phosphate in laboratory. P. polymyxa emphasized the superiority among
other species in all PGPR properties except for phosphate solubilization, whereas M. radiotolerans showed
highest amount of phosphorus solubilized in culture media. On the other hand, the results of the nematode
survey conducted on orange, Citrus sinensis L. cv Balady grown in different localities of Ismailia and Sharkia
Governorates during season 2019, revealed the presence of seven genera and species of plant-parasitic
nematodes. Among which, T. semipenetrans occurred in all examined samples (100% frequency of occurrence)
with a relatively high population density of 2330 and 2640 infected juveniles (J2) /250 g soil in Ismailia and
Sharkia Governorates, respectively. Field experiments were conducted in two different locations, at Ismailia and
Sharkia Governorates to assess the effectiveness of PGPR strains alone or combined to reduce the numbers of T.
semipenetrans during season 2019. It was found that, all treatments caused significant (P≤0.05) reduction in T.
semipenetrans population, compared to control treatment. The nematicide, Nemathorin® 10% G followed by P.
polymyxa (20 L.fed-1
) + foliarspraying of M. mesophilicm (5 L.fed-1
) gave the highest efficacy in controlling the
citrus nematode. Percentage reduction in numbers of J2/250g soil and adult females/1g roots for these treatments
in Ismailia Governorate were 91% (85.7%) and 91.4% (89.5%), respectively. While the parallel values in
Sharkia Governorate were 90% (87%) and 94% (90%), respectively. The combination of P. polymyxa and
humic acid (20 L.fed-1
) with foliar spraying of M. mesophilicm (5 L.fed-1
) gained the third position. All
treatments increased the fruit yield compared to control treatment. The highest percentages of increase were
determined with Nemathorin (160% and 206%) followed by P. polymyxa + Humic acid (20 L.fed-1
) + foliar
spraying of M. mesophilicum (155.7% and 193%) and M. radiotolerans + P. polymyxa + foliar spraying of M.
mesophilicum (153% and182%) in Ismailia, and Sharkia Governorates respectively.