Detailed guide on how to survey, record, and evaluate disease severity levels in rubber plantations, including sampling methods, disease grading, and calculation of infection indices for accurate plantation health monitoring.
1. Timing of Disease Assessment
Table of Contents
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Conduct surveys only during the period when diseases appear and cause damage (i.e., the disease season).
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Preferably assess the plantation during stages when trees are most susceptible to infection.
2. Sampling and Survey Method
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Select 3–5 sampling points per plot, arranged diagonally or in a zigzag pattern.
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The number of trees and disease grading scales vary depending on the disease type (see Annex Table 6).
Annex Table 6. Number of Survey Trees and Disease Rating Scales
| Disease Type | No. of Survey Points | Trees per Point | Total Trees | Disease Scale (0–5) |
|---|---|---|---|---|
| Powdery mildew | 3–5 | 10 | 30–50 | 0–5 |
| Leaf fall (rainy season) | 3–5 | 10 | 30–50 | 0–5 |
| Leaf blackening | 3–5 | 10 | 30–50 | 0–5 |
| Corynespora leaf fall | 3–5 | 10 | 30–50 | 0–5 |
| Tapping panel canker | 3–5 | 30–35 | 150–175 | 0–5 |
| Pink disease | 3–5 | 30–35 | 150–175 | 0–5 |
| Botryodiplodia | 3–5 | 30–35 | 150–175 | 0–5 |
3. Disease Rating and Description
3.1 Powdery Mildew, Leaf Blackening, and Corynespora – Based on Leaf Symptoms
| Scale | Severity Level | Description |
|---|---|---|
| 0 | None | No visible disease symptoms. |
| 1 | Very mild | A few small lesions or oil spots visible under close observation. |
| 2 | Mild | Lesions cover ≤12.5% of leaf area. |
| 3 | Moderate | Lesions cover 12.5–25% of leaf area. |
| 4 | Severe | Lesions cover 25–50% of leaf area. |
| 5 | Very severe | Lesions cover >50% or leaves drop. |
Note:
Evaluate on individual shoots, selecting the middle leaflet from 5 leaves per shoot. If the middle leaf has fallen, rate as grade 5.
3.2 Powdery Mildew – Based on Whole Canopy Observation
| Scale | Severity | Canopy Symptoms |
|---|---|---|
| 0 | None | No visible disease. |
| 1 | Very mild | Tiny white or oily spots, visible only at close range. |
| 2 | Mild | Scattered white spots; ≤10% leaf drop. |
| 3 | Moderate | Many white spots, leaves slightly curled or deformed; 10–25% leaf drop. |
| 4 | Severe | Dense fungal growth, yellowing and curling leaves; 25–50% leaf drop. |
| 5 | Very severe | Heavy fungal growth, severe deformation, >50% leaf drop. |
3.3 Corynespora Leaf Fall – Based on Canopy Observation
| Scale | Severity | Canopy Symptoms |
|---|---|---|
| 0 | None | No diseased leaves. |
| 1 | Very mild | A few infected leaves, visible only on close inspection. |
| 2 | Mild | Several infected leaves, easily visible; <5% leaf drop. |
| 3 | Moderate | Moderate infection (grades 2–3); 5–25% leaf drop. |
| 4 | Severe | Widespread infection (grades 3–4); 25–50% leaf drop. |
| 5 | Very severe | Heavy infection (grades 3–5); >50% leaf drop. |
3.4 Leaf Fall Disease (Rainy Season)
| Scale | Severity | Visual Range | Leaf Condition | Leaf Drop on Ground |
|---|---|---|---|---|
| 0 | None | Normal foliage | No yellowing | None |
| 1 | Very mild | Visible only up close | Slight yellowing | Very few leaves |
| 2 | Mild | Easily visible up close | Scattered yellow leaves | Few leaves on ground |
| 3 | Moderate | Visible from a distance | ~25% leaf loss | Noticeable layer |
| 4 | Severe | Visible from a distance | 25–50% leaf loss | Thin ground layer |
| 5 | Very severe | Visible from a distance | >50% leaf loss | Ground fully covered |
Note:
The proportion of yellow or green fallen leaves is a key criterion for evaluating rainy-season leaf fall intensity.
3.5 Tapping Panel Canker
| Scale | Severity | Symptoms |
|---|---|---|
| 1 | Very mild | Small dark streaks on the tapping line. |
| 2 | Mild | Streaks combine to cover <1/8 of tapping length. |
| 3 | Moderate | Lesions expand and merge, covering 1/8–1/2 of tapping cut. |
| 4 | Severe | Cankers spread over 25–50% of the tapping panel, with yellowish exudation. |
| 5 | Very severe | Lesions occupy >50% of tapping surface; bark rot and oozing present. |
3.6 Pink Disease (Corticium salmonicolor)
| Scale | Severity | Symptoms | Treatment Prognosis |
|---|---|---|---|
| 1 | Very mild | White fungus margin; early infection; easily cured. | Very easy to cure |
| 2 | Mild | Pale pink fungus, visible lesions; canopy green. | Easy to cure |
| 3 | Moderate | Pink fungal growth, bark blistering, latex exudation. | Curable if treated promptly |
| 4 | Severe | Dark pink lesions, bark decay, heavy latex flow, leaf discoloration. | Difficult to cure |
| 5 | Very severe | Rotten bark, copious latex, yellow leaves, vigorous basal shoots. | Not curable |
3.7 Botryodiplodia Disease
| Scale | Severity | Symptoms |
|---|---|---|
| 1 | Very mild | Small scattered lesions <5 mm; cover ≤10% of bark below branching point. |
| 2 | Mild | Lesions cover 10–25% of bark surface. |
| 3 | Moderate | Lesions cover 25–40% of bark; cracks or latex exudation visible. |
| 4 | Severe | Lesions cover 40–75% of bark; cracking and peeling observed. |
| 5 | Very severe | Lesions >75% of bark area; bark softens, detaches easily, heavy latex flow. |
4. Classification of Infection Severity
Using Average Disease Grade (CBTB) and Disease Index (CSB%)
| Average Grade (CBTB) | Disease Index (CSB%) | Severity |
|---|---|---|
| 0 | 0 | None |
| ≤ 1 | ≤ 20 | Very mild |
| >1–2 | 20–40 | Mild |
| >2–3 | 40–60 | Moderate |
| >3–4 | 60–80 | Severe |
| >4 | >80 | Very severe |
5. Survey Frequency
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During the disease season, organize monthly disease surveys to track the progression and evaluate the effectiveness of control measures.