Effect of temperature,relative humidity,leaf wetness and leaf age on <Emphasis Type="Italic">Spilocaea oleagina</Emphasis> conidium germination on olive leaves |
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Authors: | Friday O Obanor Monika Walter E Eirian Jones Marlene V Jaspers |
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Institution: | (1) Bio-Protection and Ecology Division, Lincoln University, P.O. Box 84, Lincoln, Canterbury, New Zealand;(2) HortResearch, P. O. Box 51, Lincoln, Canterbury, New Zealand |
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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. |
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Keywords: | Cycloconium oleagina Olea europaea Peacock spot |
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