PI-005: Chapter 4 – Pathology

Table of Contents

• 4.1 Objective
• 4.2 Definitions
• 4.3 General Pathology
• 4.4 Bacterial Diseases
• 4.5 Fungal Diseases
• 4.6Viral Diseases
• 4.7 Phytoplasmas
• 4.8 Nematodes
• Appendix 4-1 Important Potato Pests
• Appendix 4-2 Reference Material
• Appendix 4-3 Potato Disease Web Sites

4.1 Objective

Identification of potato diseases plays an important role in the seed potato certification system. Inspectors must be able to recognize and identify many major potato diseases. To do this, inspectors must understand the fundamentals of plant pathology and apply them under field conditions.

A tuber/plant disease of potato is the result of an interaction between a host (the potato) and a pathogen (bacterium, fungus, virus, mycoplasma, nematode, or environmental conditions) that impairs productivity or usefulness of the crop. The potato is susceptible to many diseases, some of which are widespread and others which are localized. Potatoes are usually vegetatively propagated by planting potato seed pieces, sets or whole tubers. This method of propagation lends itself to the introduction of potato pathogens from one area to another. As well, pathogens may overwinter in stored tubers which may be replanted the next year.

The purpose of this chapter is not to write a plant pathology text; there are many excellent reference books, fact sheets, and web pages available to the inspector. In fact, Appendix 4-2 and 4-3 contain samples of some reference materials and web pages that can be a starting point for inspectors. Rather, this chapter should be used as a general guide to the diseases that an inspector will be looking for in seed potato certification.

4.2 Definitions

BRR: Bacterial Ring Rot; the disease caused by the bacterial pathogen Clavibacter michiganensis subsp. sepedonicus;

Crop: Any Breeders Selection Seed Potatoes, or a variety and class of seed potatoes, growing in an aseptic environment, a protected environment or in one or more fields of a farm unit;

Cull: Potatoes discarded or rejected as being worthless due to size, shape, injury, disease, etc.;

Farm unit: A single tract or a number of separate tracts of land under the same or different tenure operating as a single unit which uses common equipment, facilities or storage for the production and marketing of seed potatoes under the control of a grower;

Grower: A person, cooperative, corporation or partnership that grows seed potatoes;

Host: A plant that furnishes a suitable medium for the development of a pathogen or parasite;

Hypha: The threadlike fungal filaments forming the hyphae, which make up the mycelium;

Mycelium: The vegetative part or body of a fungus made up of a mass of hyphae;

Nematode: Microscopic roundworms which include several parasitic species;

Non-quarantine pest: A pest that is not a quarantine pest for an area (IPPC, ISPM 5, 2001);

Pathogen: An entity, factor or organism capable of producing disease;

Pest: Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products (IPPC, ISPM 5, 2001);

PLRV: Potato Leaf Roll Virus;

PSTVd: Potato Spindle Tuber Viroid;

PVA: Potato Virus A;

PVM: Potato Virus M;

PVS: Potato Virus S;

PVX: Potato Virus X;

PVY: Potato Virus Y;

Quarantine pest: A pest of potential economic importance to the endangered area; not yet present there or present but not widely distributed and being officially controlled (IPPC, ISPM 5, 2001)

Seed potato: A tuber, or any part of a tuber, that is certified according to the Seed Regulations Part II for seed-reproduction purposes;

Regulated non-quarantine pest: A non-quarantine pest whose presence in plants affects the intended use of those plants with an economically unacceptable impact; is regulated by the importing contracting party (IPPC, ISPM 5, 2001);

TRV: Tobacco rattle virus; also known as corky ring spot or spraing;

Tuber: The short fleshy underground stem bearing buds or eyes;

Vector: An agent, insect, human or other mechanism able to transmit disease;

Virus: Submicroscopic infective particles containing protein and nucleic acid that are pathogenic and multiply within plant cells;

Zero tolerance:

1. In respect of a disease, the requirement of the absence of a disease in a plant, or in any part thereof (including a tuber), and
2. In respect of a varietal mixture, the requirement of the absence of the mixture of two or more varieties.

4.3 General Pathology

Pathology is derived from the Latin word pathos which means suffering state. Two important terms used in pathology are etiology (i.e. cause) and epidemiology (i.e. development). Knowledge of these terms will assist the inspector when accessing journal articles, research papers or specific disease information.
There are three basic requirements that form the basis of the disease triangle:

• A susceptible host,
• A causal agent, and
• A favourable environment.

All three of these components are necessary for disease to occur. The interaction of the host, causal agent (i.e. pathogen) and environment over time are all needed for the development of disease.

Disease can be defined as: change from the normal physiology of a plant which results from continuous interaction with an agent (biotic or abiotic) resulting in the production of symptoms and the reduction in the economic or aesthetic value of the plant. Field grown potatoes are usually vegetatively propagated by the use of cut seed pieces or whole seed. The progeny are subject to the same pathogens as the mother plant which can lead to introduction of disease from one area to another.

A few abiotic symptoms are caused by environmental factors such as temperature extremes, moisture extremes, nutritional extremes, air pollution and chemical injury. Where one or more of these abiotic symptoms are shown to have impaired seed quality, inspectors should be concerned and should document their observations. In seed potato production, damage from such abiotic agents can result in both crop yield and economic losses. Below is one of the most commonly used classification system that links disease to major causal agents is shown below. This two-tier system is simple and easy for inspection staff to follow.

I Pathogenic (biotic) diseases incited by:

• Bacteria
• Fungi
• Viruses
• Phytoplasma/Mycoplasma and/or related forms
• Nematodes

II Abiotic diseases incited by:

• Low temperature plant injury
• High Temperature
• Unfavourable oxygen relations
• Unfavourable soil-moisture relations
• Lightning, hail, and wind (plant injury)
• Mineral excesses
• Mineral deficiencies
• Chemical injury

Inspectors should refer to reference material for a description of diseases and pests. In particular, Appendix 4-1 lists important potato pests. Further, Sections 4.4 - 4.8 of this chapter also provide detailed discussions on several pathogenic causal agents. Note that throughout these sections, an asterisk (*) denotes quarantine pests.

4.4 Bacterial Diseases

Most bacteria are beneficial because they break down organic tissues and put nutrients back into the soil. Relatively few of them attack living organisms. However, pathogenic bacteria can cause serious problems, under favourable conditions and several are of economic importance to potatoes.

Most bacteria that cause plant diseases survive in the soil but develop in host plants. In some cases, the population in the soil declines quickly in the absence of a host, and therefore does not contribute to the maintenance of the disease from season to season. However, bacteria can overwinter in vegetative material. In seed potatoes, infected tubers serve as a means of disease spread.

Bacteria can be spread from one plant to another by being splashed by rain or water onto neighbouring plants or by moving short distances in wet soil. They can also be spread longer distances on machinery and in irrigation water. Some bacteria persist in plant debris and can be spread by the wind. The most efficient means of spreading bacterial diseases over long distances is the transport of infected plant material.

In potatoes, the most common method of spreading bacteria is by infected tubers. Several bacterial diseases are of particular importance in potato inspection. They are the following:

• Bacterial Ring Rot - Clavibacter michiganensis subsp. sepedonicus
• Brown Rot - Ralstonia solanacearum*
• Blackleg and soft rot - Pectobacterium spp. and Dickeya spp.
• Common Scab - Streptomyces scabies

4.5 Fungal Diseases

Fungi are present everywhere in the world. Common fungi include mushrooms, mildews and breadmolds. Fungi can be beneficial in that they breakdown dead plants and animals and release the nutrients back to the soil. They can also be harmful to plants and cause disease. Several diseases of economic importance to potatoes are caused by fungi.

Fungi, unlike plants, do not produce their own food. They get their nutrients either by breaking down other organisms after their death or by living off other organisms as parasites.

Fungi can use one or several of the reproductive methods listed below to propagate:

• The fungus can produce spores through sexual reproduction. In many fungi, oospores are the survival structure. They can remain dormant for a long period or until conditions become favourable.
• The fungus can produce spores asexually. There are a number of different kinds of asexual spores. They vary in longevity and dispersal styles.
• The mycelium itself can be transported on plant material such as potatoes. (asexual)
• The hyphae can mass together in a sclerotium, a sturdy dormant structure that can survive long periods of drought or cold. (asexual).

Fungi can spread in several ways. Over wintering spores and sclerotia can be found in cull potatoes, on potatoes in storage or on unharvested tubers remaining in the field. For most fungi, conditions of high moisture and/or high humidity are required for the production and germination of spores. Spores can be transported by wind or moved with soil. Fungi can survive in crop debris, soil, on infected tubers and other solanaceous hosts (e.g. nightshade, tomato, peppers, eggplants, etc.)

Some fungal diseases that are important to seed potato inspection include:

• Potato wart - Synchytrium endobioticum*
• Late blight - Phytophthora infestans
• Verticillium wilt - Verticillium albo-atrium
• Early blight - Alternaria solani
• Rhizoctonia canker - Rhizoctonia solani
• Fusarium dry rots - Fusarium spp.
• Fusarium wilts - Fusarium spp.
• Leak - Pythium ultimum
• Silver scurf - Helminthosporium solani

4.6 Viral Diseases

The potato, like all vegetatively reproduced crops, may be infected by a great number of virus and virus-like diseases, which are carried through the tubers to the next generation and can be spread rapidly during the season by mechanical means or via insect and nematode carriers called 'vectors'.

Virus and virus-like organisms are grouped according to their mode of transmission for better understanding of the disease complexities. An understanding of the pathogens involved and their relationship with their host and their vectors, will enable inspectors to reach more rational decisions, advise growers on appropriate control practices and render his/her job more interesting.

The modes of transmission of viruses may be:

• Vegetatively;
• Mechanically;
• By true seed;
• By insects; such as:
  • Aphids
  • Leafhoppers
  • Other (e.g. beetles, moths)
• By other modes (e.g. fungi, nematodes).

Some virus and virus-like diseases that are important to seed potato inspection include:

• PLRV - Potato Leafroll Virus
• PSTVd - Potato Spindle Tuber Viroid
• PVA - Potato Virus A
• PVM - Potato Virus M
• PVS - Potato Virus S
• PVX - Potato Virus X
• PVY - Potato Virus Y
• TRV - Tobacco Rattle Virus*

4.7 Phytoplasmas

Phytoplasmas, formerly known as mycoplasmas, are organisms similar in structure to bacteria. Phytoplasmas are not mechanically transmitted but rather are transmitted by leafhoppers and perhaps other insects. They are acquired by leafhoppers from weeds and other crop plants, but not from potatoes. Phytoplasmas can survive in the tubers, but if planted useful tubers are not usually produced.

Some phytoplasmas that are important to seed potato inspection include:

• Aster yellows, purple top, stolbur, haywire;
• Witches broom; and
• Psyllid yellows.

4.8 Nematodes

Nematodes are tiny roundworms that live in soil, water and plant tissues. They are the most numerous multicellular animals on earth and are present in nearly all ecosystems. Nematode pests of agricultural crops feed exclusively on plants, with most species attacking roots and below ground plant structures.

Nematodes pathogenic to potatoes occur in all climates and can cause serious crop losses, but much of this damage is either unrecognized or attributed to other causes. Due to the fact that nematodes attack roots and tubers, no diagnostic symptoms appear on aboveground parts of the plant except for stunted top growth resulting from poor root systems. Low nematode populations in the soil cause little or no top symptoms, but may reduce tuber yields.

Nematodes of importance to seed potato inspection are:

• Golden nematode - Globodera rostochiensis*
• Lesion nematode - Pratylenchus spp.
• Pale cyst nematode - Globodera pallida
• Potato rot nematode - Ditylenchus destructor
• Root-knot nematode - Meloidogyne spp.
• Columbia root-knot nematode - Meloidogyne chitwoodi

(* Denotes quarantine pest)

Appendix 4-1 Important Potato Pests

Appendix 4-2 Reference Material

• Aphids Infesting Potatoes in Canada: A Field Guide. M.E. MacGillivary. 1979. Agric. Can. Publ. 1676/E
• Color Atlas of Post-Harvest Diseases and Disorders of Fruits and Vegetables, Vol. 2: Vegetables. 1992. Wolf Publishing Ltd., Fla. USA.
• Compendium of Potato Diseases. W. R. Stevenson, et al. ed. 2001. APS Press, St. Paul, Minnesota
• Diseases and Pests of Potatoes. Hodgson, W.A., D.D. Pond and J. Munro. 1974. Can. Dep. Agric. Publ. 1492/E. (Revised)
• Diseases and Pests of Vegetable Crops in Canada. R.J.Howard, J.A.Garland and W.L.Seaman. 1994. The Canadian Phytopathological Society/Entomological Society of Canada
• Integrated Pest Management for Potatoes in the Western United States. Anonymous. 1986. Univ. Calif. Div. Agric. Natl. Resources, Publ. 3316
• Plant Protection Manual. Inspector's Training Course. Seed Potatoes. Plant Health and Production Division. Camelot Court, 59 Camelot Court, Ottawa, Ont
• Potato Diseases. Potato Marketing Board, Research and Development. 1980. 50 Hans Cresent, Knightsbridge, London, U.K.
• Potato Health Management. Randall C. Rowe, ed. 1993. APS Press, St Paul, Minnesota

Appendix 4-3 Potato Disease Web Sites

• Agriculture and Rural Development
• Government of Prince Edward Island
• Canadian Food Inspection Agency - Potatoes
• Canadian Food Inspection Agency - Plant Protection
• FarmAssist Syngenta
• Potato Disease Problems