Top 6 Factors Influencing Disease Cycle of Rice | Plant Disease

The following points highlight the top six factors influencing disease cycle of rice. The factors are: 1. Conidial Germination 2. Appressoria Formation and Host Penetration 3. Dispersal and Spread 4. Lesion Growth 5. Sporulation 6. Latent Period.

Disease Cycle of Rice: Factor # 1. Conidial Germination:

Leaf wetness is required for all of the infection sub processes; the relationship between infection and Leaf wetness is well known. If leaf wetness ends before completion of infection, the process is terminated. Increasing the leaf wetness period form 12 to 15 h is known to result in a 30% increase in infection.

The length of the leaf wetness period required for infection is influenced by temperature. Conidial germination and appressoria formation depend on the temperature of 25-28°C and RH near saturation or wetness duration.

Germination commonly begins within 3 hours of deposition if leaves are wet. Germination is delayed if conidia are exposed to a dry period, also pre infection processes are not affected by plant water stress.

Effect of temperature on spore germination of P. oryzae:

1. No germination was observed at 13°C for up to 15 hrs of incubation and reach 37% in 20 hrs.

2. At 15 and 18°C, germination started within 3 hrs reaching a maximum of 55.54 and 62.32%, respectively after 9 hrs.

3. Within a range of 20-28°C spores started to germinate within 2 hrs reaching more then 90% in 8 hrs.

Effect of sunlight on spore germination and appessorial formation of P. oryzae:

1. Sunshine strongly inhibits germination of both wet and dry spores.

2. Inhibition increased with an increase in the duration of exposure to sunlight. .

3. Inhibition was slightly more in case of wet spores.

4. The ambient temperature increase from 25 to 34°C.

5. Formation of appressoria was also greatly reduced in wet spores as compared to dry spores exposed to sunlight.

1. The shortest period of leaf wetness needed for successful infection was 12 hrs. Within a temperature range of 20-25°C where as 16hrs at 18°C.

2. Difference in the number of leaves showing infection and average number of lesion per leaf within temperature range of 20-25°C were negligible.

3. No significant difference was found in the number of lesions produced on leaves exposed to varying duration of leaf wetness (12-22 hrs.) at a particular temperature between 20 and 25°C.

4. However, number of leaves per seedling showing infection was greatest at 16hrs of leaf wetness at 20°C and 22 hrs of leaf wetness at 25°C.

Disease Cycle of Rice: Factor # 2. Appressoria Formation and Host Penetration:

Initiation of infection occurred from 5 to 7 hrs at 21 to 27°C and 8hrs at 18°C conidial deposition. Appressoria formation occurred at an average of 11 h at 24°C but is rare at temperatures greater than 28°C. Various optimum temperatures for aspersorium formation have been reported to be between 15 and 25°C.

Temperature and dew period interact to affect infection. A minimum dew period of 6-8 h will initiate infection at the optimum temperature of 25°C. In the tropic, night temperature is almost always in the optimal range. In upland rice, longer dew periods were suggested as a factor in the higher incidence of blast compared with lowland condition.

Spore germination and aspersorium formation are stimulated in plants fertilized with high levels of nitrogen. Penetration rates are lower in plants with high levels of silica, possibly as a result of the mechanical barrier attributed to accumulation of silica in the epidermis.

Application of N results in increased infection what is unclear from the literature is whether N confers a direct physiological effect on the pathogen-host interaction or only has an indirect effect by changing plant morphology and consequently, microclimate.

Disease Cycle of Rice: Factor # 3. Dispersal and Spread:

Blast conidia are commonly observed at high number over rice crops between 0000 h and 0600 h, and liberation is favored by darkness, high RH and wind speeds < 3.5 ms^-1. Dispersal gradients for blast conidia are strongly related to dominant wind directions and follow the pattern found for other passively liberated spores.

Spores may be released at temperatures above 19°C while rainfall on rice leaves is known to cause an increase in the conidial concentration over canopies. Rain, wind and air currents strongly affect the number of dispersing conidia.

In South India, Ramagalam (1966) observe the peak spore load from September to October at 4.00 a.m. and February to April at 6.00 a.m. during December with maximum around 2.00 a.m., lesser numbers in the other months and absent in May.

The maximum distance that a blast conidium can travel remains controversial, but is key to determining management strategies. The work of Kato and Sasaki (1974) remains probably the most authoritative.

Under average tropical conditions, blast conidia can be dispersed commonly within about 230 meters of the source. It is very possible that long-distance dispersal of conidia is closely related to weather patterns and low-level upper atmospheric disturbances.

This aspect of Epidemiology deserves more study in view of modern techniques for spatial analysis of weather and landscape patterns, and for monitoring of fungus and host.

Disease Cycle of Rice: Factor # 4. Lesion Growth:

Cultivars with different degrees of partial resistance to blast show differences in infection efficiency and lesion size. Plant age and leaf age, however, influence the expression of partial resistance effects; young leaves are commonly more susceptible.

Lesion size is known to be affected by temperature, with high alternating temperature of 32/25°C and 32/20°C resulting in initial rapid lesion growth, and water stress increasing the size of lesions. Blast management through cultural practices will therefore be enhanced by minimizing water stress, especially at period of rapid leaf growth.

Disease Cycle of Rice: Factor # 5. Sporulation:

Under optimum conditions, conidiophores are formed in 4-6 hrs, with one conidium formed every 40 min. The number of spores produced by any one lesion has been found highest on the sixth day after lesion appearance.

Mature lesions are capable of sporulating when relative humidity (RH) is higher than 89% (with optimum RH >93%). Maximum Sporulation occurs 7-12 days after inoculation, although Sporulation may continue at low levels for 60 days.

Sporulation is greater in lesions on leaves that were expanding when inoculated than on lesions from leaves that were fully expanded 3-4 days before inoculation Lesions produced on plants inoculated at the tillering stage has a greater sporulation potential than on plants inoculated after the formation of the panicle primordium.

In one study number of conidia per lesion varied from 4800 on IRAT13 to 33,000 on IR442-2-58. Maximum sporulation is associated with lesions having gray centers and margins changing from dark purple to brown.

Lesions on plants showing water stress symptoms may produce up to 3.5 times more conidia than those on unstressed plants. The infectious period is also prolonged as a result of pre infection water stress.

Disease Cycle of Rice: Factor # 6. Latent Period:

Successful establishment after infection has occurred depends on the age and genotype characteristic of the host, crop nutrition, and environmental factors such as temperatures and soil moisture. Latent period varied from 13-18 days at 9-11°C and 4-6 days at 26-28°C.

Computer simulation studies show that small changes in latent period can significantly change the epidemic infection rate in many path systems. In tropical and subtropical areas where blast in known to be a problem, temperature changes during the different crop season are sufficient to cause some part of the year to be more blast conducive.

The seasonal profiles for weather in the tropic and subtropics are not as static as previously assumed by blast workers. Small changes such as those shown are important enough epidemiologically to allow more targeted blast management.