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I. Fusarium Wilt

1. Cause

Caused by Fusarium oxysporum, a fungus belonging to the Deuteromycetes. The pathogen enters through root wounds or root hairs, multiplies within the xylem vessels, and blocks the vascular bundles, while simultaneously secreting toxins that obstruct water transport in the plant.

2. Onset Time

Mainly severe during the flowering and fruiting period. It is prone to outbreaks when temperatures are between 25℃ and 28℃ and the soil is moist.

3. Damage

A typical vascular bundle disease, often causing half of the plant to wilt or the entire plant to die. The disease progresses slowly, but once symptoms appear, it often leads to total crop failure.

4. Identifying Characteristics

Wilting sequence: Leaves wilt from bottom to top and from the outside in.

Vascular bundle color: When the stem base is cut open, the vascular bundles are yellowish-brown to dark brown (usually darker, like soy sauce).

Humidity manifestation: In humid conditions, a pinkish mold layer (conidia) often appears at the stem base.

Special phenomenon: Some plants will exhibit unilateral wilting (one side of the branches and leaves is normal, while the other side withers).

5. Control methods

Agricultural control: Strictly implement crop rotation (rotating with non-solanaceous and cucurbitaceous crops for 4-5 years); select disease-resistant varieties; apply lime to adjust soil pH.

Biological control: Treat the soil or dip roots with Trichoderma or Bacillus subtilis at planting.

Chemical control: In the early stages of disease, drench the roots with carbendazim, hymexazol, or metalaxyl. In severe cases, diseased plants must be removed and destroyed outside the field, and the affected area should be disinfected with lime.

II. Verticillium wilt

1. Cause

Caused by deuteromycetes such as Verticillium dahliae. This pathogen has a very wide host range and can form microsclerotia in the soil, surviving for a long time.

2. Onset Time

It mostly occurs in the mid-to-late stages of plant growth (fruiting stage). It is easily prevalent under cool conditions of 20℃-25℃, and symptoms may subside in hot weather (above 30℃).

3. Damage

It causes yellowing and drying of leaves, premature aging of plants, smaller fruits, and a severe decline in yield and quality.

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4. Identifying Characteristics

Leaf Color Change: Leaves turn yellow from bottom to top, with chlorosis occurring first at the leaf margins and between the veins, presenting a yellow-green “mosaic” appearance, similar to nutrient deficiency.

Vascular Bundle Color: When the stem is cut open, the vascular bundles are light brown to yellowish-brown (lighter than the color of Fusarium wilt).

Leaf drop symptoms: In severe cases, a large number of leaves fall off, leaving bare stems.

5. Control methods

Agricultural control: Crop rotation with gramineous crops; strictly prohibit the introduction of seeds from diseased areas; deep plowing to reduce surface inoculum.

Disease-resistant grafting: For crops such as eggplant and cotton, grafting onto wild, disease-resistant rootstocks is an effective measure.

Chemical control: Fumigate the soil with dazomet before planting. At the early stage of disease, drench the roots with carbendazim, thiophanate-methyl, or pyraclostrobin.

III. Bacterial Wilt

1. Cause

Caused by *Ralstonia solanacearum* (bacteria). The pathogen enters through root wounds, multiplies rapidly within the vascular bundles, and spreads via rainwater, irrigation water, and farm implements.

2. Time of onset

The disease is most severe during hot and humid seasons, especially after heavy rains followed by sudden sunshine in summer, when temperatures are between 30℃ and 37℃.

3. Damage

A typical case of acute bacterial wilt. The disease progresses extremely rapidly, often causing the entire plant to wither and die quickly, posing a significant threat to crops such as tomatoes, peppers, tobacco, and potatoes.

4. Identifying Characteristics

Wilting State: * **Green Wilt:** While the leaves have not yet faded and remain green, the entire plant or the upper leaves suddenly wilt due to water loss. It may recover in the morning, evening, or on cloudy days, but the entire plant will soon die.

* **Vascular Bundle Color:** When the stem base is cut, the vascular bundles turn brown.

* **Key Identification:** Cut the diseased stem horizontally and place it in a clear glass of water. After a short while, a milky white, sticky liquid (bacterial ooze) will ooze from the cut. This is the key difference between this and fungal wilt (fusarium wilt/yellow wilt).

5. Control Methods

* **Agricultural Control:** Avoid continuous cropping; use raised beds with deep furrows to prevent waterlogging; apply lime to acidic soils (adjusting the pH to above 7 can inhibit bacteria).

Biological control: Utilize antagonistic bacteria such as *Pseudomonas fluorescens* and *Bacillus subtilis* for root drenching.

Chemical control: At the initial stage of disease, immediately drench the roots with copper-based preparations (such as thiabendazole copper, copper oxychloride) or antibiotics (kasugamycin, kasugamycin). Strictly avoid flooding to prevent the spread of pathogens through water.

IV. Root Rot

1. Cause

Mostly caused by complex infection of multiple fungi such as *Pythium*, *Phytophthora*, and *Fusarium*. Primarily damages the root system (taproot and fibrous roots).

2. Time of Onset

Can occur throughout the entire growth period, but is particularly prevalent during the seedling stage and when soil moisture is high and soil temperature is low (spring or rainy season).

3. Damage

Causes root rot, loss of water and nutrient absorption capacity, leading to slow growth, stunted growth, yellowing, and in severe cases, death of the entire plant.

4. Distinguishing Features

Root Symptoms: The epidermis of the taproot or lateral roots turns brown and rots; the root bark is easily peeled off, exposing the xylem; fibrous roots are reduced, and no new roots appear.

Above-ground Symptoms: Plant growth is slow; leaves turn yellow and thin; the plant wilts during the day and recovers in the morning and evening; after several days of this cycle, the entire plant dies.

Stem Base: The stem base is usually intact (distinguishing it from stem base rot); the lesions are mainly concentrated in the roots.

5. Control Methods

Agricultural Control: Select well-drained plots; increase the application of well-rotted organic fertilizer and phosphorus and potassium fertilizer to promote strong root systems; use raised-ridge cultivation.

Seed Treatment: Before sowing, coat seeds with seed dressing agents such as metalaxyl-mancozeb-fludioxonil.

Chemical Control: In the early stages of the disease, drench the roots with hymexazol, metalaxyl, or cymoxanil hydrochloride, focusing on the roots.

V. Stem Base Rot

1. Cause

Mainly caused by fungi such as Rhizoctonia solani and Pythium. 1. **Pathogen primarily infects the stem base near the ground.**

2. **Onset Time:** Most common in seedlings, but also prevalent in mature plants under high temperature, high humidity, and dense canopy conditions.

3. **Damage:** Directly damages the vascular tissue at the stem base, causing damping-off (seedling stage) or complete plant collapse (mature stage), a major cause of missing seedlings and gaps in rows.

4. **Distinguishing Features:**
**Affected Area:** Lesions are concentrated at the stem base, 3-5 cm above the ground surface. The roots are usually intact or only slightly infected.

**Lesion Morphology:** Initially, dark brown, oval, sunken lesions appear at the stem base, gradually expanding to encircle the stem. The affected area is often accompanied by spiderweb-like white mycelia or brown sclerotia (characteristic of Rhizoctonia solani).

**Plant Condition:** Diseased plants die upright (without lodging) or break off at the stem base and collapse.
5. Control Methods

Agricultural Control: Ensure good ventilation and reduce humidity during seedling cultivation to avoid excessively high seedbed temperatures; avoid planting too deep during transplanting.

Seed Treatment: Disinfect seedbed soil or treat seeds with difenoconazole before sowing.

Chemical Control: In the early stages of disease, spray or drench the base of the stem with thifluzamide, azoxystrobin, or metalaxyl. Remove diseased plants promptly to reduce the source of infection.

Summary and Key Differentiation Points

1. Observe the wilting speed:

Fastest: Bacterial wilt (acute onset, with bacterial ooze).

Slower: Fusarium wilt (half-wilt, pink mold layer).

Slow: Verticillium wilt (yellowing and leaf drop, onset at low temperatures).

Gradual: Root rot (poor growth, gradual death).

Breakage: Stem base rot (stem base rot, entire plant collapses).

2. Observe the affected area:

Root rot: Root rot. Stem base rot: Rot at the base of the stem (near the ground).

Wilting/Verticillium wilt/Bacterial wilt: Rot of the vascular bundles (internal discoloration).

3. Observe the color of the vascular bundles and secretions:

Wilting: Dark brown (soy sauce color), no white sap.

Verticillium wilt: Light brown, no white sap.

Bacterial wilt: Brown, with white bacterial ooze (water test).

In terms of control strategies, these diseases all follow the principle of “prevention first, comprehensive control.” Because treatment is difficult after onset, the focus should be on soil disinfection, selection of disease-resistant varieties, crop rotation, and early root irrigation. Once large-scale necrosis occurs in the field, chemical agents are often insufficient to recover the losses.