Protozoan Plant Pathogens:

Protozoa are unicellular eukaryotic organisms, and while they are not as common as fungi, bacteria, or viruses, certain species of protozoa can infect plants, causing significant damage. The protozoa associated with plant diseases are often flagellates and sporozoa, though they are less well-studied compared to other plant pathogens. Here are the key examples:

1. Plasmodiophora brassicae (Clubroot Disease)

• Pathogen Type: Protozoan (Plasmodiophora spp. – a type of slime mold-like protozoan).
• Host Plants: Plants in the Brassicaceae family, including cabbage, cauliflower, mustard, broccoli, and other cruciferous plants.

Symptoms:

• • Infected plants exhibit swollen, distorted, and club-shaped roots. These abnormal roots reduce the plant’s ability to absorb water and nutrients from the soil.
  • Yellowing of leaves (chlorosis) and poor growth are common, and infected plants may eventually wilt and die.
  • The disease leads to reduced crop yields, as the roots’ ability to anchor and absorb nutrients is severely compromised.

Mode of Transmission:

• • Plasmodiophora brassicae is soil-borne, and its resting spores, known as zoospores, can survive in the soil for several years.
  • The zoospores are spread by contaminated water, soil, and tools.
  • They infect the roots of the host plant, where the spores germinate, infect the root tissue, and lead to the characteristic clubbing and deformities.

Control Measures:

• • Crop rotation with non-host plants.
  • Soil solarization to reduce spore load in the soil.
  • Use of resistant plant varieties where available.

2. Phytomonas spp. (Coconut and Oil Palm Diseases)

• Pathogen Type: Protozoan (Flagellates from the genus Phytomonas, belonging to the family Trypanosomatidae).
• Host Plants: Primarily affects oil palm, coconut palms, and some other tropical plants such as coffee and cassava.

Symptoms:

• • In coconut palms, symptoms of hart rot include yellowing and browning of older leaves, which spreads to younger leaves.
  • Unripe nuts fall prematurely, and inflorescences turn black. Infected trees may eventually die after significant decline in overall health.
  • In oil palms, the disease known as sudden wilt leads to browning of the lower leaves, leading to ash grey leaves and premature death of the palm.
  • Chlorosis and wilting are common, along with malformation of phloem tissue in some cases.

Mode of Transmission:

• • Insect vector transmission: The disease is often transmitted by pentatomid insects such as Lincus and Ochlerus, which feed on the infected plant’s vascular system.
  • Root grafts can also transmit the disease between plants.

Control Measures:

• • Insect vector management through the use of insecticides or biological control.
  • Regular monitoring and removal of infected plants to reduce disease spread.

3. Phytomonas leptovasorum (Phloem Necrosis of Coffee)

• Pathogen Type: Protozoan (Flagellate from the genus Phytomonas).
• Host Plants: Affects Coffea liberica and Coffea arabica.

Symptoms:

• • Infected coffee plants exhibit sparse yellowing of leaves, followed by leaf drop. Only the young top leaves remain on the branches, leading to a bare appearance.
  • The disease causes the death of the tree, typically within 3-6 weeks after infection.
  • Infected trees have necrotic phloem tissue, and the flagellates can be traced from the roots upward to the trunk.

Mode of Transmission:

• • The disease is transmitted through root grafts and by insect vectors (specifically, the pentatomid insect Lincus sp.).
  • Horizontal transmission occurs via infected root contact between plants.

Control Measures:

• • Grafting with clean, disease-free rootstock.
  • Managing insect populations that serve as vectors.

Phanerogamic Parasites (Flowering Plant Parasites)

Phanerogamic parasites are higher plants (seed-producing plants) that parasitize other plants. These parasites extract nutrients and water from their host plants and often cause damage through their feeding. Some of the most well-known phanerogamic plant parasites include dodder (Cuscuta spp.) and witchweed (Striga spp.).

1. Cuscuta spp. (Dodder)

• Pathogen Type: Phanerogamic parasitic plant (flowering plant).
• Host Plants: Affects a wide range of plants, including tomato, potato, beans, sunflower, lettuce, and many other crops.
• Symptoms:
  • Dodder is a leafless parasitic plant that attaches to the host plant using specialized structures called haustoria, which penetrate the host’s vascular tissues.
  • It causes stunted growth in the host, yellowing of leaves, wilting, and death of the host plant if left uncontrolled.
  • Dodder rapidly spreads to neighboring plants, forming dense mats over the host’s surface.
• Mode of Transmission:
  • Dodder seeds can spread through wind, water, insects, or mechanical transmission (e.g., through contaminated equipment or tools).
  • The parasite germinates and attaches to the host plant by sending out tendrils to connect to the vascular system, where it begins to siphon nutrients.
• Control Measures:
  • Manual removal of infected plants and weeds.
  • Crop rotation with non-host plants.
  • Use of resistant plant varieties if available.

2. Striga spp. (Witchweed)

• Pathogen Type: Phanerogamic parasitic plant (flowering plant).
• Host Plants: Mainly affects maize, sorghum, millet, rice, and other cereal crops.
• Symptoms:
  • Striga attaches to the host plant’s roots using haustoria and steals nutrients, leading to yellowing, wilting, and stunted growth.
  • Affected plants often exhibit reduced vigor, poor root development, and lower crop yields.
  • Striga is known for causing yield losses in regions of sub-Saharan Africa, making it a major agricultural pest.
• Mode of Transmission:
  • The seeds of Striga are dispersed by wind, water, and by insects. Once they land on a host, they germinate and infect the root system.
• Control Measures:
  • Crop rotation with non-host plants (e.g., legumes) is effective.
  • Striga-resistant varieties are being developed.
  • Use of herbicides or biocontrol agents to reduce the spread of Striga.

3. Rafflesia spp. (Corpse Flower)

• Pathogen Type: Phanerogamic parasitic plant (flowering plant).
• Host Plants: Tetrastigma species (specific woody vines).
• Symptoms:
  • Rafflesia is a parasitic flower that grows inside the tissues of its host plant. It does not cause the same type of “disease” symptoms as other plant pathogens but can reduce the vigor of the host by absorbing nutrients from it.
  • Rafflesia is famous for producing the world’s largest flower, but it is also a nutrient thief.
• Mode of Transmission:
  • The seeds are dispersed by insects, especially flies, which are attracted to the foul odor of the flowers.