Effect of temperature, relative humidity, leaf wetness and leaf age on Spilocaea oleagina conidium germination on olive leaves

Abstract

The effects of temperature, relative humidity (RH), leaf wetness and leaf age on conidium germination were investigated for Spilocaea oleagina, the causal organism of olive leaf spot. Detached leaves of five 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 RH levels (60, 80 and 100%) were tested. Results showed that percentage germination decreased linearly in proportion to leaf age (P<0.001), being 58% at 2 weeks and 35% at 10 weeks. A polynomial equation with linear term of leaf age was developed to describe the effect of leaf age on conidium germination. Temperature significantly (P<0.001) affected frequencies of conidium germination on wet leaves held at 100% RH, with the effective range being 5 to 25°C. The percent germination was 16.1, 23.9, 38.8, 47.8 and 35.5% germination at 5, 10, 15, 20 and 25°C, respectively, after 24 h. Polynomial models adequately described the frequencies of conidium germination at these conditions over the wetness periods. The rate of germ tube elongation followed a similar trend, except that the optimum was 15°C, with final mean lengths of 175, 228, 248, 215 and 135 μm at 5, 10, 15, 20 and 25°C, respectively after 168 h. Polynomial models satisfactorily described the relationships between temperature and germ tube elongation. Formation of appressoria, when found, occurred 6 h after the first signs of germination. The percentage of germlings with appressoria increased with increasing temperature to a maximum of 43% at 15°C, with no appressoria formed at 25°C after 48 h of incubation. Increasing wetness duration caused increasing numbers of conidia to germinate at all temperatures tested (5-25°C). The minimum leaf wetness periods required for germination at 5, 10, 15, 20 and 25°C were 24, 12, 9, 9 and 12 h, respectively. At 20°C, a shorter wetness period (6 h) was sufficient if germinating conidia were then placed in 100% RH, but not at 80 or 60%. However, no conidia germinated without free water even after 48 h of incubation at 20°C and 100% RH. The models developed in this study should be validated under field conditions. They could be developed into a forecasting component of an integrated system for the control of olive leaf spot.

Factors Affecting Infection and Disease Development on Olive Leaves Inoculated with Fusicladium oleagineum

“…According to this function, the optimum temperature for infection was 12.4°C. This value is within the range of 10 to I5°C determined by other authors to be optimal for germination of F. oleagineum conidia (29,30,34). The inoculation of olive plants at the highly favorable daily temperature regime of 20/12°C revealed a linear relationship between severity of infection and wetness period.…”

“…As noted earlier, previous reports about the influence of wetness duration and temperature on olive scab come from field observations (9) or from spore germination studies (29,30,32,45). but seldom from experiments in which plants were inoculated and maintained under controlled conditions (24,28).…”

“…Most of these studies indicate that infection of olive leaves by F. oteagineum requires long wetness periods as regulated by temperature, although the studies differ regarding the details of wetness duration and temperature. Modugno (28) reported minimum wetness periods for infection of olive leaves of 36, 24 to 36, 18, and 14 h at 5, 12, 18, and 24°C, respectively. In germination studies on olive leaves, Saad and Masri (32) indicated that wetness periods of 48, 24, and 36 h were required for appressorium formation at 16, 20, and 24°C, respectively, but wetness periods of only 9 to 24 h have been reported as requirements for conidial germination (29). Longer and shorter wetness durations than those found in our study were reported for naturally infected plants under field conditions (4,24,28,36), but these observations should be considered with caution because the exact time of infection and the effects of cultivar susceptibility or environmental conditions were often unknown.…”

“…The nature of ontogenic resistance in other pathosystems has been associated with various physical and chemical features of the leaf cuticle or with host substances induced or favored by infection (25). In olive, researchers have reported a lower rate of conidium germination on old than on young leaves (29,34), which would reduce the susceptibility of old leaves in terms of numbers of lesions produced. In addition, a reduced rate of conidium germination, germ tube elongation, and cuticle penetration has been identified as a resistance mechanism in young leaves of the olive cultivar Lechín de Sevilla (3),…”

Effect of Environmental Factors on Mycelial Growth and Conidial Germination of Fusicladium eriobotryae, and the Infection of Loquat Leaves

“…Temperature requirements obtained are similar to those of other pathogens in the genus Venturia, such as V. inaequalis (2), V. nashicola (18), F. carpophilum (16), and F. oleaginum (23). These results increase the current knowledge of the epidemiology of this pathogen and of its life cycle, which are both essential steps to develop a disease prediction system.…”