Plant diseases in Guam [typescript]

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Title: Plant Diseases in Guam By: Dr. R.G. Beaver

00165 PLANT DISEASES

All plants are subject to disease if the conditions are favorable and the inoculum of the causal agent is present on a susceptible plant.

The principle causes of plant disease are in order of size viruses, mycoplasma, bacteria, fungi and nematodes.

VIRUSES These causal agents are microscopic in size and can be viewed only with an electron microscope.

There are two general virus types.

STYLET BORNE – This type is best represented by the viruses which are transmitted by aphids. The virus particles are acquired by these insects when they feed on infected plants. Transmission is usually immediate, that is to say at the time of the next feeding by the aphid carrying the viruses particles. A commonly occurring example of this virus types is papaya mosaic. Another unique feature of this virus type is that most can be mechanically transmissed.

CIRCULATIVE – This type is best represented by the viruses which are transmitted by leaf hoppers. The virus particles are acquired by these insects at time of feeding which often requires several leaf probings. Once acquired the virus incubates in the insect for a period of time before transmission can take place. Most of these viruses are not mechanically transmissed but must have the insect vector. The corn viruses are examples of this type.

MYCOPLASMA This group of causal agents are known to produce a yellow type symptom on affected plants. Like the viruses they are microscopic in

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-2size, although distinctly different in shape, can be viewed only with an electron microscopic.

These too are acquired by insects, aphids, at time of feeding. The insect is immediately infective and will transmit the causal agent at the next feeding. Bunchy top bananas is example of this disease in the tropics.

Viruses and mycoplasma in general do not survive outside the living host so destruction of infected plant material is an effective method of preventing causal agent spread to healthy plants. There are no effective chemical controls for viruses and mycoplasma.

BACTERIA Many bacteria are known, however, only four genera – Corynebacterium, Pseudomonas, Xanthomonas and Erwinia, are considered plant pathogens. This group of causal agents can be viewed with a special light microscope. The common practice for bacteria identification however is the use of special or selective agar media. Bacterial cells are extracted from the infected plant parts and seeded on the selective media were distinct features are allowed to develop for the purpose of specific identification.

Initial infection takes place in several different ways. Bacteria can be seed borne becoming contaminated at the time fruit is fermented, like tomato bacterial canter - Corynebacterium Michiganese. Infected plants, which initially show no visible symptoms, are of course a common method of sprading a bacterial infection. Once infection symptoms develop, particularly on aerial parts of the plant, bacterial cells can be readily spread by water droplets from rain or over head sprinklers.

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-3Probably the most serious bacterial plant pathogen, because it is soil borne, is bacterial wilt. The pathogen or causal agent persist in the soil in the absensce of a susceptible host plant. Pseudomonas solanacearum is known to occur on tomatoes, peppers, eggplant, edible ginger, and bananan.

The genus Erwinia contains both pathogens and soft rot or decay bacteria. Often the decay that occurs after mechanical injury to the plant part in contact with the soil in side to this bacterium.

The genus Xanthomonas has a number of important plant pathogens including bacterial fruit spot of tomato and citrus canker. The only effective chemical control is a combination of Dithane M – 45 plus Tribasic copper sulfate. Clean seed and resistant varieties are more effective controls for the bacterial plant pathogens.

FUNGI These plant pathogens are broadly separated into those which survive only on the living plant and those which affect the living plant and survive on plant debris.

Phycomycetes – Mycelia is non-septate and fruiting structures may be asexual or sexual. Important fungi in this group are the water molds, Pythuim and Phytophothora, the downy mildews, Pernospora, Plasmopara, Bremnia, and the white rusts like Albugo candida.

Ascomycetes – Mycelia is septate and specialized fruiting structures, perithecia, produce the spores in a series of four borne in a sac within the fruiting structure. Important fungi in this group are the powdery mildews. Erysiphe, Unicinula, the sclerotia bearing fungi, cottony rot, Sclerotinia sclerotiorum, southern Blight, Sclerotium rolfsii; black streak of banana, Mycospharella fijiensis, Uredinales – These fungi are unique in that all produce distinct fruiting structures on affected plant

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-4parts. These belong to a group of fungi that survive on a living host often an alternate host. Important rusts are found in the genus Puccinia, Uromyces.

Fungi Impertecti – This group of fungi includes the highest number of known plant pathogens. All are produced asexually but may have a (sexual) perfect stage which belongs to another group.

Cercospora – leaf spot and fruit spot.

Alternaria – leaf spot and fruit spot.

Oidium – Often found on all plant parts.

Colletotrichum – Leaf spot and fruit spot.

Phomopsis – Basal stem and fruit spot.

Rhizoctonia – Crown and root rot, soil rot of fruit.

Cylindrocladium – Collar or crown rot.

Ascochyta – Leaf, petiole and fruit spot.

Phyllostictina – Leaf spot and fruit spot.

Septoria – Leaf spot.

Fusarium – Vascular root rot or wilt.

Botrytis – Flower blight and a decay fungus Phoma – Leaf spot and fruit spot.

Sphaceloma – Scab of leaves.

NEMATODES Living microscopic worms that attack the roots of plants. Nematodes are roughly divided into those that actually invade the roots, endoparasites and those that feed from outside the root, ectoparasites. This latter group has genera that include some that may enter the root tissue but do not cause the same type of damage by those in the endoparasite group.

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-5Endoparasitic nematodes The best known genus includes the rootknot nematode, Meloidogyne and the cyst nematode Heterodera.

Rootknot nematodes cause an actual swelling of the affected roots with common examples found in those vegetables known as the Solonaceae, tomato, pepper and eggplant. They are however not confined to this group of plants but may be found on many different types of crop plants. All the nematode have stylets or feeding apparatus that resemble a hypodermic needle with which they extract the vital juices from the plant cells. This in turn produces a variety of plant symptoms including stunting, general leaf chlorosis and poor growth response when applying nutrients in whatever form.

Ectoparasitic nematodes This group of nematode is identical to the previous group except for the habit of feeding. All with few exceptions are free living in the soil surrounding the roots. Two of the best known genera are the burrowing nematode, Radoholus and the Reniform nematode, Rotylenychus. Both of these nematodes can be a problem on banana and have been known as serious pests of pineapple.

Nematode control involves the use of chemicals known as nematocides but populations can be depressed by adding organic composts and wood chips to the soil. Because nematode movement is very restricted the major source of contamination is infested soil or plants.

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PLANT DISEASES ON GUAM VEGETABLES A) Beans – Phaseolus vulgaris 1) Angular leaf spot – Isariopsis griseola 2) Anthracnose – Colletotrichum lindemuthianum 3) Bacterial blight – Xanthomonas phaseoli 4) Crown and root rot – Rhizoctonia solani 5) Damping-off – Pythium ultimum 6) Powdery mildew – Oidium sp.

7) Rust – Uromyces phaseoli 8) Virus – Bean mosaic B) Beans – Vigna sesquipedalis 1) Bacterial spot – Pseudomonas syringae 2) Damping-off – Rhizoctonia solani 3) Leaf spot – Mycospharella cruenta 4) Powdery mildew – Erysiphe polygoni 5) Southern blight – Sclerotium rolfsii C) Bitter Melon – Momordia charantia 1) Powdery mildew – Oidium sp.

D) Cassava – Manihot esculenta 1) Anthracnose – Gloeosporium manihotis 2) Leaf spot – Cercospora henningsii 3) Root knot – Meloidogyne sp.

4) Root rot - Rhizoctonia solani

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2 E) Cabbage – Brassica sp.

1) Bacterial black rot – Xanthomonas campestris 2) Bacterial leaf spot – Pseudomonas maculicola 3) Bacterial soft rot – Erwinia carotovora 4) Blackleg – Phoma lingam 5) Club root – Plasmodiophora brassicae 6) Damping-off – Rhizoctonia solani 7) Downey mildew – Peronospora parasitica 8) Grey leaf spot – Alternaria brassicae 9) Grey mold – Botrytis cinerea 10) Leaf mold – Heterosporium variabile 11) Leaf spot – Gercosporella brassicae 12) Nematodes – Heterodera sp.

Pratylenchus sp.

Meloidogyne sp.

13) Powdery mildew – Oidium sp.

14) Rot, cottony – Sclerotinia sclerotiorum 15) Rot, soft – Rhizopus stolonifer 16) Rot, root – Phymatotrichum omnivorum 17) Southern blight – Sclerotium rolfsii 18) White spot – Mycosphaerella brassicicola 19) Yellow wilt – Fusarium oxysporum F) Chinese leeks – Allium porrum 1) Root rot – unknown G) Corn – Zea mays 1) Bacterial wilt – Xanthomonas stewartii 2) Corn smut – Ustilago maydis

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3 Corn rust – Puccinia polysora 3) Corn stripe virus – Viral 4) Leaf blight – Helminthosporium turticum H) Cucumber – Cumumis sativus 1) Antracnose – Colletotrichum lagenarium 2) Bacterial leaf spot – Pseudomonas lachrymans 3) Bacterial wilt – Erwinia tracheiphila 4) Cucumber mosaic virus – Viral 5) Crown and root rot – Rhizoctonia solani 6) Downey mildew – Pseudoperonospora cubensis 7) Powdery mildew – Oidium sp., Shpaerotheaca fuliginea, Erysiphe cichoxacearum 8) Leaf spot – Cercospora cucurbitae 9) Root knot – Meloidogyne sp.

10) Southern blight – Sclerotium rolfsii 11) Wilt – Fusarium oxysporum I)

Eggplant – Solanum melongena 1) Bacterial wilt – Pseudomonas solanacearum 2) Fruit rot – Phomopis vexans 3) Damping-off – Rhizoctonia solani 4) Root knot – Meloidogyne incognita 5) Southern blight – Sclerotium rolfsii

J)

Melon – Citrullus sp. and Cucumis melo 1) Anthracnose – Colletotrichum lagenarium 2) Bacterial soft rot – Erwinia carotovora 3) Belly rot – Phytophthora sp., Sclerotinia sclerotiorum 4) Damping-off – Rhizoctonia solani 5) Gummy stem blight – Mycosphaerella citrullina

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4 6) Cucumber mosaic virus – Viral 7) Powdery mildew – Oidium sp.

8) Wilt – Fusarium oxysporum 9) Root knot – Meloidogyne sp.

K) Okra – Hibiscus esculentus 1) Southern blight – Sclerotium rolfsii 2) Verticillium wilt – Verticullium albo-atrum L)

Onion – Allium sp.

1) Purple blotch – Alternaria porri 2) Rust – Puccinia allii 3) Southern blight – Sclerotium rolfsii

M)

Peanut – Arachis hypogaea 1) Leaf spot – Alternaria sp.

2) Rust – Puccinia arachidis 3) Scab – Sphaceloma arachidis

N) Pepper – Capsicum sp.

1) Anthracnose – Glomerella cingulata 2) Bacterial leaf spot – Pseudomonas syringae 3) Bacterial spot – Xanthomonas vesicatoria 4) Bacterial wilt – Pseudomonas solanacearum 5) Damping-off – Rhizoctonia sp., Pythium sp.

6) Southern blight – Sclerotium rolfsii 7) Virus – Spotted wilt 8) Virus – Potato Y 0)

Radish – Raphanus sp.

1) Black rot – Xanthomonas campestris 2) Bacterial soft rot – Erwinia carotovora 3) Damping-off – Rhizoctonia solani

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5 4) Crown rot – Sclerotinia sclerotiorum 5) White rust – Albugo candida P) Sweet potato – Ipomoea batatas 1) Anthracnose – Elsinoe batatas 2) Soil stain – Monilochaetes infuscans 3) Soil rot – Streptomyces ipomoea 4) Southern blight – Sclerotium rolfsii 5) Grey mold – Botrytis cinerea 6) Stem wilt – Fusarium oxysporum Q) Taro – Colacasia sp. and Xanthosoma sp.

1) Corm rot – Physiological 2) Dasheen mosaic virus – Viral 3) Leaf blight – Phytophthora colocasiae 4) Southern blight – Sclerotium rolfsii R) Tomato – Lycopersicon esculentum 1) Bacterial canker – Corynebacterium michiganense 2) Bacterial soft rot – Erwinia carotovora 3) Bacterial wilt – Pseudomonas solanacearum 4) Damping-off- Rhizoctonia sp., Pythium sp.

5) Early blight – Alternaria solani 6) Late blight – Phytophthora infestans 7) Root knot – Meloidogyne sp.

8) Southern blight – Sclerotium rolfsii 9) Wilt – Fusarium oxysporum 10) Wilt – Verticillium albo-atrum S) Yams – Dioscorea batatas 1) Leaf rust - Unknown

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6 FRUIT DISEASES A) Avocado – Persea sp.

1) Anthracnose - Glomerella cingulate 2) Nematode – Radolphus similis 3) Root rot – Phytophthora cinnamomi B)

Banana – Musa sp.

1) Black leaf streak – Mycosphaerella fijiensis 2) Bunchy top – Mycoplasma 3) Burrowing nematode – Radopholus similis 4) Freckle – Phyllostictinia musarum 5) Lesion nematode – Pratylenchus musicola 6) Moko – Pseudomonas solanacearum 7) Panama wilt – Fusarium oxysporum f. cubensis 8) Root knot – Meloidogyne sp.

9) Sigatoga – Mycosphaerella musicola

C) Beetlenut – Areca catechu 1) Sooty mold – Capnodium sp.

D) Breadfruit – Artocarpus altilis 1) Anthracnose – Colletotrichum sp.

2) Fruit rot – Phytophthora palmivora 3) Soft rot – Rhizopus stolonifer E) Citrus – Citrus sp.

1) Albinism – Aspergillus flavus 2) Brown rot – Phytophthora citrophthora 3) Citrus canker – Xanthomonas citri 4) Citrus scab – Elsinoe fawcetti

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7 5) Damping-off – Rhizoctonia solani 6) Greasy spot – Glomerella cingulata 7) Gummosis – Phytophthora sp.

8) Melanose – Diaporthe citri 9) Nematode – Radopholus similis 10) Penicillium molds – Penicillium sp.

11) Southern blight – Sclerotium rolfsii 12) Viral – Exocortis, Psorosis, Tristeza F) Coconuts – Cocos nucifera 1) Bud rot – Phytophthora palmivora 2) Leaf spot - Pestalotiopsis polmarum 3) Sooty mold – Capnodium sp.

4) White threadblight – Corticium penicillatum G) Grape – Vitus rotundifolia 1) Rust – Phakopsora vitis H) Guava – Psidium sp.

1) Anthracnose – Glomerella cingulata 2) Nematode – Radopholus similis 3) Root rot – Phytophthora sp.

I)

Mango – Mangifera indica 1) Anthracnose – Glomerella cingulata 2) Algal leaf spot – Cephaleuros virescens 3) Powdery mildew – Oidium Mangifera 4) Sooty mold – Capnodium sp.

H) Papaya Carica papaya 1) Anthracnose – Glomerella cingulata 2) Damping-off – Phythium aphanidermatum

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8 3) Leaf spot – Asperisporium caricae, Cercospora sp.

4) Powdery mildew- Oidium sp.

5) Mosaic virus – Viral 6) Ring spot virus – Viral 7) Root rot – Phymatotrichum omnivorum 8) Root & Stem blight – Phytophthora palmivora 9) Stem rot – Fusarium sp., Phytophthora sp.

K) Pineapple – Ananas comosus 1) Crown rot – Phytophthora infestans 2) Mealybug wilt – Virus 3) Root knot – Meloidogyne sp.

L) Soursop – Annona muricata 1) Root rot – Phytophthora infestans