Bacterial Leaf Streak Of Rice
This article is co-authored by Lorelvn Joy Turnos, a research associate of Pioneer Hi-Bred Philippines based in Mindanao. For further information on this article, send your inquiries to mark.nas@pioneer.com.
In this issue, we will talk about another important bacterial disease of rice known as bacterial leaf streak (BLS), which is caused by the bacterium Xanthomonas oryzae pv. Oryzicola. The disease is less popular and less invasive than bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae, but is generally more prevalent and infectious during rainy seasons.
Symptoms and mechanisms of damages of the two bacterial diseases are almost similar. Hence, it is very important for the rice growers to gain skills and expertise in distinguishing what particular causal organisms are attacking their fields, and what types of symptoms are manifested by the crops. BLS infection usually comes in earlier than BLB, wherein disease symptoms become noticeable 40 to 45 days after seeding (DAS).
ECONOMIC IMPORTANCE
The disease may occur in both lowland and upland rice areas. It is reported to be widely distributed in tropical and subtropical Asia, including Thailand, India, Philippines and Indonesia, and has recently become a significant problem in Western Africa. The disease is not reported to occur in temperate countries such as Japan and Korea.
The bacterium is a seedborne pathogen, and it is difficult to control once introduced. Bacterial streak may develop during any development stage of the plant, but it is mostly destructive during the tillering to booting stages. It can result in yield losses through the reduction in thousand grain-weights, and can cause losses up to 20 percent to 30 percent depending on the rice variety, plant age, and climatic conditions.
BLS is highly important during very wet seasons and where high rates of nitrogen (N) are used. Yield reduction is generally less significant in less fertile soils, especially when low N rates are applied. Losses of 5 to 30 percent have been reported in India, while in the Philippines, losses are not considered significant in either wet or dry seasons.
MECHANISM OF DAMAGE
Potential sources of bacterial inoculum include infected planting materials, volunteer rice plants, infected straws, and weed hosts. Pathogenspenetrate the leaves through natural-openings of the plants (stomata and hydathodes), leaf injuries, artificial Wounds, and surface damage often caused by insects bites, winds, etc.
Highest occurrence of disease infection usually takes place during midday wherein leaf stomata are fully opened. The pathogen multiplies in the substomatal cavity and masses of bacteria develop in the parenchyma cells in between the veins of the leaves. Once inside the vascular system, the bacterium multiplies and moves in both directions.
Spread within a crop occurs by mechanical contact and in rain and irrigation water. Under favorable warm wet conditions, rapid and severe disease development can occur. The bacterium can persist from season to season on infected leaves and leaf debris, but is unable to survive in non-sterile soil.
SYMPTOMS
Initial symptoms of BLS include the formation of narrow, dark green, water-soaked, interveinal streaks of various lengths on the leaf blades. The lesions enlarge and streaks eventually turn yellow to orange or yellowish-gray and translucent with numerous milky to yellow beads of bacterial exudates. Individual plants may develop a pronounced yellowing of the tips of new leaves if infection occurs early in the season.
As the disease progresses, streaks become more diffuse and coalesce, then eventually turn brown to grayish-white, causing leaves to die. High temperatures, high humidity, and rainy weather favor the development of the disease.
BLS vs BLB
Proper field diagnosis based on symptoms alone may be difficult. Thus, direct observation of the bacterium is necessary for confirmation. Bacteria frequently enter the damaged feeding sites associated with lepidopterous leaf rollers, leaffolders, and hispa beetles.
The most obvious difference of the diseases is the shape of the edges of the lesions: BLS remains straight or linear, while it is wavy in BLB. Tips of the leaves infected with bacterial streak become droopy, while BLB-infected leaves remain erect. A more pronounced mass of rustic yellow color above the canopy is associated with BLS infection.
FACTORS FAVORING DISEASE DEVELOPMENT
a. Strong winds, heavy rains, and deep water
b. High temperature and humidity
c. Presence of the bacteria on leaves and in water
d. Sources of inoculum (volunteer plants, infected straws, weeds, etc.)
e. Over fertilization (excessive use of N)
f. Poor handling during transplanting (wounded leaves)
g. Closer plant spacing (high frequency of tissue contacts among plants)
MANAGEMENT STRATEGIES
a. Reducing the amount of inoculum by practicing field sanitation. Destroying rice ratoons, volunteer seedlings, crop stubbles, infected straws, and weeds can minimize the inoculum at the beginning of the cropping season. Fallowing the field and allowing drying thoroughly can kill the bacterial pathogens that may have survived in the soil and plant residues.
Maintaining proper drainage which will also prevent water stagnation and will provide good aeration is one of the best preventive options to minimize disease occurrence.
b. Reducing the spread of the disease by avoiding seedling damages. This is done by proper handling of the crops during transplanting and various fertilizer and chemical application activities.
These injuries will provide point of entries for the causal pathogens. Hence, these will allow rapid occurrence of the disease. Infection spreads through direct plant-to-plant contact and through water (strong winds and heavy rain).
Use of copper fungicides may help prevent disease occurrence and stop further disease spread. The Dupont product Kocide® (containing copper hydroxide) is effective against bacterial leaf diseases of rice such as leaf blight and streak. Fungicide application must be done not later than 40 DAS. Beyond this, the bacteria must have taken their place inside the leaf.
c. Reducing the susceptibility of the plants to infection. This is done by planting resistant varieties, application of balanced levels of plant nutrients, proper plant spacing, and seeding rate.
Excessive amount of N can worsen disease infection. Likewise, a variety resistant to BLB is also probably resistant to BLS. Dry season is the best time to grow hybrid rice as rice crops are highly susceptible to BLS and other diseases during wet season.
By Mark Nas