AGRI 41 Lab | Causes of Plant Diseases (no plant culture pics)

Plant Pathology

Study of plant pathogens and diseases they cause in plants

Disease

Any malfunctioning of host cells and tissues that continues from continuous irritation by a pathogen, or an environmental factor that leads to development of symptoms

Pathogen

Agents that causes disease which is primarily defined as biotic or living

Parasite

Organism that depends partly or wholly on another organism (host) for its resources

Plant Disease

Is a phenomena due to specific agents (primarily biotic) that causes physiological disorder that is detrimental to the plant

What are the four pathogen groups?

Mycology, Bacteriology, Nematology, Virology

Diseases caused by true fungi

Rice blast, Rice Sheath Blight, Corn Leaf Spot, Peanut Leaf Spot, Peanut Rust, Corn Rust, Panama Wilt of Banana, Black Sigatoka of Banana, Mango Anthracnose, Steam End Rot of Mango, Okra Leaf Mold, Purple Blotch of Onion

Rice Blast

Magnaporthe oryzae (Pyricularia oryzae)

Rice Blast

• Airborne

• Elliptical lesions resembling a spindle shape, which tapers towards each end.

• Center of the spot is whitish to gray with brown to reddish brown margin

Rice Sheath Blight

Rhizoctonia solani

Rice Sheath Blight

• Soil-borne

• Initial oval to ellipsoidal greenish gray lesions on leaf sheath

• Older lesions with grayish-white centers with tan to dark brown irregular

Corn Leaf Spot

Bipolaris maydis

Corn Leaf Spot

• Begin as small diamond-shaped lesions

• Develops into elongated or rectangular brown spots

Peanut Leaf Spot

Cercospora arachidicola

Peanut Leaf Spot

• Dark brown to black spots on leaves

• Usually with yellow halo

Peanut Rust

Puccinia arachidis

Peanut Rust

• Pin-head size, brick-red pustules on underside of the leaf

Corn Rust

Puccinia polysora

Corn Rust

• Round, cinnamon brown pustules on underside of the leaf

Panamana Wilt of Banana

Fusarium oxysporum f. sp. cubense

Panama Wilt of Banana

• Yellowing of the older leaves

• Pseudostem has brick red to brown streaks

• Wilting of leaves at the petiole base

Black Sigatoka of Banana

Mycosphaerella fijiensis (Pseudocercospora fijensis)

Black Sigatoka of Banana

• Brown streaks with bright yellow margin, some with white center

• Lesions enlarge, becoming fusiform or elliptical, and darken to give the characteristic black streaking of the leaves

• Blighted area at the leaf tip and margins where the spots have joined together

Mango Anthracnose

Colletotrichum gloeosporioides

Mango Anthracnose

• Circular brown spots on leaves, collapsed spots, sometimes resulting to shot-hole

• Black rot on fruit skin or surface

Stem End Rot of Mango

Lasiodiplodia theobromae

Stem End Rot of Mango

• Dark brown to black rot develop from the stem end of the fruit

Okra Leaf Mold

Cercospora abelmoschi

Okra Leaf Mold

• Black moldy growths appear on the lower leaf surface

• Reddish discoloration is observed on the upper leaf surface

Purple Blotch of Onion

Alternaria porri

Purple Blotch of Onion

• Starts with water-soaked lesions with white center on the leaves

• Progress as brown lesions with purple margin

Diseases caused by fungal-like oomycetes

Damping-off of seedlings, Corn Downy Mildew, Gabi (Taro) Leaf Blight, White Rust of Kangkong

Damping-Off of Seedlings

Pythium spp.

Damping-Off of Seedlings

• Water-soaked rot of root and stem

• Above soil plants parts topple down or wilt

Corn Downy Mildew

Peronosclerospora spp.

Corn Downy Mildew

• White to yellow chlorotic streaks or stripes along leaf veins

• White growth of structures associated with fungi is seen at the underside of the leaf

• Systemic disease

Gabi (Taro) Leaf Blight

Phytophthora colocasiae

Gabi (Taro) Leaf Blight

• Circular, zonate, brown to purplish brown lesions on leaves

White Rust of Kangkong

Albugo sp.

White Rust of Kangkong

• White or yellow-creamy pustules on stems, flowers, or lower leaf surface

• Affected parts may curl

Diseases caused by bacteria

Bacterial wilt of solanaceous crops, bacterial blight of rice, soft rot of vegetables, bacterial blight of anthurium, citrus canker, huanglongbing/citrus greening

Bacterial Wilt of Solanaceous Crops

Ralstonia solanacearum

Bacterial Wilt of Solanaceous Crops

• Rapid wilting and death of plants

• Lower leaves turn yellow, sometimes on one side of the plant or one side of a branch. This is followed by leaf and stem wilting

Bacterial Blight of Rice

Xanthomonas oryzae pv. oryzae

Bacterial Blight of Rice

• Lesions develop as water-soaked to yellow-orange stripes on leaf blades or leaf tips with wavy margin and progress toward the leaf base

• Old lesions turn yellow to grayish white with black dots

Soft Rot of Vegetables

Pectobacterium carotovorum

Soft Rot of Vegetables

• Water-soaked lesions which rapidly spread and develop into deterioration of the tuber (or fruit) into a slimy, foul-smelling mass

Bacterial Blight of Anthurium

Xanthomonas axonopodis pv. dieffenbachiae

Bacterial Blight of Anthurium

• Yellowed (chlorotic), water-soaked lesions along the leaf margins that grow rapidly to form dead (necrotic) V-shaped lesions

Citrus Canker

Xanthomonas citri

Citrus Canker

• Blister-like lesions on leaves and fruit darken to tan or black and develop a water-soaked margin with a yellow halo surrounding it

• Lesions may appear raised and corky or scabby surrounded by a water-soaked margin

Huanglongbing/Citrus Greening

Candidatus Liberibacter spp.

Huanglongbing/Citrus Greening

• Blotchy mottle leaves

• Reduced fruit size

• Stunted growth

• Root decline and dieback

• Yellow shoot

Diseases caused by phytoplasma

Rice Orange Leaf, Cassava Witches’ Broom

Rice Orange Leaf

Candidatus Phytoplasma asteris

Rice Orange Leaf

• Orange-colored leaves which later roll inward and desiccate

Cassava Witches’ Broom

Candidatus Phytoplasma luffae

Cassava Witches’ Broom

• Shortening of nodes with dense clumping of stunted leaves, resembling a witch’s broom

Diseases caused by nematodes

Root Knot of Solanaceous Crops, Toppling/Blackhead Disease of Banana, Corn Root Lesion

Root Knot of Solanaceous Crops

Meloidogyne incognita

Root Knot of Solanaceous Crops

• Above ground: stunting and chlorosis

• Below ground: numerous enlarged roots termed as galls or knots

Toppling/Blackhead Disease of Banana

Radopholus similis

Toppling/Blackhead Disease of Banana

• Above ground: uprooting (toppling) of banana, especially when fruiting or during strong winds, because of destroyed anchor roots

• Below ground: black, circular lesions on rhizomes or roots

Corn Root Lesions

Pratylenchus spp.

Corn Root Lesions

• Above ground: stunted growth and leaf chlorosis

• Below ground: lesions and necrosis

Diseases caused by viruses

Tobacco Mosaic, Rice Tungro, Abaca/Banana Bunchy Top, Papaya Ringspot, Tomato Leaf Curl, Rice Grassy Stunt, Citrus Tristeza

Tobacco Mosaic

Tobamovirus tabaci

Tobacco Mosaic

• Intermingled pattern of light green and dark green areas on leaves with distinct boundaries between the yellow light and green

Rice Tungro

Bacilliform:
Tungrovirus oryzae

Spherical:
Waikavirus oryzae

Rice Tungro

• Stunting and reduced tillering

• Yellow-orange leaf discoloration

Abaca/Banana Bunchy Top

Banana:

Babuvirus musae

Abaca:
Babuvirus abacae

Abaca/Banana Bunchy Top

• Stiff, narrow, short upright or erect leaves that crowd together at the apex of stems or pserudostems

• Dark green streaks on leaf veins

• Chlorotic edges on the leaves

Papaya Ringspot

Potyvirus papayanuli

Papaya Ringspot

• Mosaic on the leaves: intermingled pattern of light green and dark green areas on leaves with distinct boundaries between the yellow light and green

• Ringspot on the fruit: concentric rings or irregular almost circular chlorotic or dark green lines on fruits

Tomato Leaf Curl

Begomovirus solanumphilippinense

Tomato Leaf Curl

• Upward or downward curling of leaves

• Stunted growth

Rice Grassy Stunt

Tenuivirus oryzabrevis

Rice Grassy Stunt

• Narrow, stiff and pale yellow leaves

• Profuse tillering

Citrus Tristeza

Closterovirus tristezae

Citrus Tristeza

• Die-back

• Stem pitting

• Yellowing of leaves

Diseases caused by viroids

Potato Spindle Tuber, Cadang-Cadang Disease of Coconut

Potato Spindle Tuber

Pospiviroid fusituberis

Potato Spindle Tuber

• Severely reduced growth of infected plants

• Tubers are small, elongated, and cracked

• ‘eyes’ are more pronounced than normal

Cadang-Cadang Disease of Coconut

Cocadviroid cadangi

Cadang-Cadang Disease of Coconut

• Non-necrotic yellow spots on leaves

• ‘brooming’ symptom caused by limited development of the lamina in palms

• Rounding of nuts and reduction in the husk

Diseases caused by phanerogams

Loranthus philippinensis, Cuscuta sp.

Loranthus philippinensis

• Hemiparasite or partial stem parasite

• With leaves but lacks true roots

Cuscuta sp.

• True parasite

• Yellow to orange leafless vine strands around the host

• Stem wind itself around host