Olive leaf spot: epidemiology and control

Abstract

The overall goal of this research was to improve the understanding of the environmental factors affecting development of olive leaf spot (OLS) in New Zealand olive groves and to develop strategies for the disease management.

The effects of temperature, relative humidity (RR), leaf wetness and leaf age on conidium germination were investigated for Spilocaea oieagina, the causal organism of olive leaf spot. Detached leaves of five leaf ages (2, 4, 6, 8, and 10 weeks after emergence), six different temperatures (5, 10, 15, 20, 25, and 30°C), eight wetness periods (0, 6, 9, 12, 18, 24, 36, and 48 h), and three RR levels (60, 80 and 100%) were tested. Results showed that percentage germination decreased linearly in proportion to leaf age (P<0.00 1). Temperature significantly (P<0.001) affected conidium germination, with the effective range being 5 to 25°C and the optimum being 20°C. The rate of germ tube elongation followed a similar trend, with the optimum being 15°C. Formation of appressoria, when found, occurred 6 h after the first signs of germination and their frequency followed the same temperature trends with a maximum of 43% at 15°C and none formed at 25°C. The minimum leaf wetness periods for germination at 5, 10, 15, 20, and 25°C were 24, 12, 9, 9, and 12 h, respectively. Studies on the prepenetration and infection processes on olive leaves by S. oieagina, using scanning electron microscopy, showed that the penetration pegs allowed direct penetration of the cuticle, but could not penetrate the host through the trichomes.

A study of the effects of temperature, leaf age, conidial concentration, continuous and interrupted leaf wetness periods and relative humidity (RR) during the dry periods on whole plants showed that OLS severity decreased (P<0.001) with increasing leaf age at the time of inoculation. The severity of OLS also increased as inoculum concentration increased from 1.0 X 102 to 2.5 X 105 conidia per mL at all five temperatures (5-25°C), with most lesions formed at 15°C and least at 25°C. The minimum leaf wetness periods for infection at 5, 10, 15, 20, and 25°C were 18, 12, 12, 12, and 24 h, respectively. Infection frequencies during wet periods interrupted with dry periods of 70 and 100% RH depended on the RH and length of the dry period and the length of the initial wet period, with the lowest OLS severity after 12 h wetness and 12 h at 70% RH.

Sporulation on olive leaf spot lesions followed similar trends over the same temperatures and moisture conditions, with maximum sporulation at 15°C and 100% RH but none at 25°C and 70% RH. Lesion expansion was affected (P<0.001) by the same conditions, with 15°C and continuous wetness being optimum and very little expansion occurring at 25°C.

Sequence analysis of rDNA (lTSl-5.8S-ITS2), restriction fragment length polymorphism (RFLP), and universally primed-polymerase chain reaction (UP-PCR) techniques were used to study the genetic structure of S. oleagina populations using 98 isolates from New Zealand olive groves along with one isolate each from Australia and Italy. Alignment of the cloned sequences of four New Zealand isolates with the isolates from Australia and Italy showed great similarity with similarity indices for ITS1, ITS2 and 5.8S coding regions of the isolates being 96-100, 99.8-100 and 99-100%, respectively. RFLP analysis revealed no differences between the New Zealand isolates but UP-PCR found low levels of gene and genotypic diversity in all populations. Analysis of molecular variance (AMOVA) showed small but significant (P = 0.001) variations among regions, and most of the molecular variability (87%) was found within populations.

Greenhouse studies of the fungicides, boscalid, captan, carbendazim, copper hydroxide, copper sulphate, difenoconazole, dodine, kresoxim-methyl, and a kresoxim-methyllcopper hydroxide mixture, showed that all significantly (P<0.001) reduced OLS severity, with highest efficacy when applied up to 3 days pre- and 3 days post-inoculation. In field trials, most of the fungicides reduced disease incidence when applied twice in spring and autumn. Of the fungicides tested, copper sulphate and a mixture of kresoxim-methyl/copper hydroxide were the most effective, reducing disease severity by 85 to 96% and 63 to 93%, respectively.

This research has provided three models relating climatic parameters to OLS development that focus on conidium germination, infection and sporulation models which should be field-tested with the more promising fungicides. They could then be incorporated into a disease forecasting system, which may be used in conjunction with a spray schedule that could improve disease control for New Zealand growers by optimising the number and timing of fungicide applications.