Sustainable Control of Plant Disease

Sustainable control of plant disease is important for everyone who wants a flourishing garden. The presence or absence of plant disease depends on three factors:

• the presence of a pathogen (disease causing agent)
• the environment
• a susceptible host¹.

Disease will take hold if plant species are vulnerable either via their genetic makeup or because their  environment is less than ideal.  Plants will be more resistant if temperature, rainfall, humidity, soil composition, pH and carbon dioxide levels are optimal.  Conditions of high rainfall, high humidity and high soil moisture favour most plant diseases.  Given current and future changes in climate due to global warming, gardeners need to be aware of likely increases in plant diseases and how to deal with them².

Plant Pathogens

Disease-causing agents include fungi, bacteria, viruses, protozoa (single celled organisms), parasites such as nematodes

Fungi

Although fungi play a vital role in decomposing dead organic matter, they cause the majority of plant diseases.  Rather than deriving energy from sunlight like plants, they get energy from breaking down organic matter and some species kill the tissue of their host in order to feed upon it.

Fungi are a separate kingdom of living things, quite separate from animals, plants, bacteria or viruses and number around 100,000.1 But only about 19,000 fungal species cause plant disease³.

Fungi are able to enter plants by direct penetration of their tissue, through natural openings via their stomates or through tissue wounds which arise from pruning, harvesting, hail, other diseases or insect damage. These infections come from seeds or soil which are contaminated, from nearby plants or plant waste and from weeds.  They can be spread by wind, water, animals including insects, humans, tools, and other plant material⁴.

They mainly reproduce by spores which can remain dormant in soil or other media for very long periods of time when conditions become favourable.

Common fungal diseases include apple scab (Venturia inaequalis), peach leaf curl (Taphrina deformans), powdery mildews, downy mildew, black spot on roses, sooty mould, cinnamon fungus (Phytophthora cinnamomi).

Symptoms of fungal infection include necrosis (death of plant tissue), stunting of plant parts or entire plants or, alternatively, excessive growth of plant parts or entire plants.

Examples of necrosis are root rot, basal stem rot, damping off, canker, anthracnose, leaf spots, scab, blight, soft and dry rots.

Other disease conditions which can be caused by fungi include clubroot, galls, warts, witch’s brooms and wilts although bacteria or viruses may sometimes be involved.

Bacteria

Most plant bacteria are opportunistic and the majority break down dead tissue. They are spread and enter plant

tissue in ways similar to those used by fungi and multiply in the spaces between cells.  Plant damage may be a result of destruction of plant tissue by enzymes or by toxins that the bacteria  release.

Symptoms of bacterial disease include cause galls, blights, wilts, soft rots, root nodules, scabs and leaf spots.

Viruses

Plant viruses are not cells and cannot exist or multiply unless they have a host.  They cannot enter plant cells by themselves and they must do so if they are to replicate, move through the plant and cause disease.  They are spread and enter plants in ways similar to those used by fungi and bacteria⁴.

Symptoms of plant viruses include stunting, discoloration, mottling, mosaics, chlorosis, ringspot, wilt, leafroll, distortion, tumours, necrotic lesions and leafspots.  Viral disease cannot be cured so prevention is the only strategy.

Nematodes

These very tiny, worm-like animals are parasites which pierce plant cells with their mouth parts. .  They are often microscopic, containing only about 1000 or fewer cells. They may inject enzymes to pre-digest cell material before feeding. Common names of nematodes tend to be those of their symptoms i.e. root knot, cyst, root lesion, spiral, burrowing, bulb and stem reniform, dagger, bud and leaf and pine wilt. Other symptoms may be total plant decline or yellowing or fewer or smaller leaves⁵.

Addressing plant disease

Avoiding chemical sprays and powders should be top of mind for gardeners who wish to protect the natural environment and themselves.  Disease problems can be minimized by a range of approaches.

Prevention

As the hackneyed saying goes “Prevention is better than the cure” and can be achieved by implementing the approaches outlined below.

Crop Rotation

Rotating vegetable and fruit crops can help prevent buildup of pathogens in the soil.

Disease Exclusion

Only use certified disease-free seed and try to use plant varieties that are resistant or tolerant to viruses and bacteria.

When purchasing seedlings or obtaining plants or cuttings from friends look for those that are healthy and vigorous. Refuse to purchase plants showing any sign of disease or poor health.

Garden Hygiene

Disinfecting garden tools between working on different plants is essential to prevent transmission of any disease-causing organisms or their spores. So clean secateurs (or other cutting instruments) with bleach or alcohol (methylated spirits) each time. The most significant risk is when working on a series of the same species and is especially relevant for virus transmission since viruses tend to be host specific.

Moisture Control

Moist/humid environments favour fungal and bacterial growth. So improve drainage and airflow around plants and do not water leaves of plants if they are unlikely to dry out before evening.

Monitor for Disease Vectors

Keep an eye out for potential vectors of disease and remove them.  For example, aphids can transmit viruses such as lettuce mosaic virus and papaya ringspot virus which infect cucumbers, melons, zucchini and pumpkin.

Soil Solarisation

As well as being a useful technique to kill weeds and weed seeds, solarisation can also kill fungi, bacteria, nematodes and insects.  Destroying these pathogens reduces the chances of plant infection. A thin clear plastic film can be placed over moist soil and left in place for 4 – 6 weeks during a hot period of the year⁶.

Ways to Treat Disease Once it is Present

Removal and Destruction of Plant Material

A plant virus infection is for the life of the plant. There are no cures for plant viruses. Any plants that are heavily infested with viruses or other untreatable diseases should be completely removed including as much of the root system and infested soil as possible. All plant material should be sealed  in plastic and either placed in rubbish bins or burned.

If a plant fungal or bacterial infection is mild and confined to a limited area it could be removed by pruning it put well below the site of infestation.  Dispose of pruned or fallen plant material in sealed plastic bags as described for viruses. Do not compost them since compost heaps are unlikely to reach temperatures high enough to destroy fungal spores.

Try Organic Treatments

Many “ home” remedies for plant diseases have been described using household substances like corn meal, milk, baking soda, soap, cinnamon, chamomile tea and many more.  Few, however, have been subjected to rigorous testing so SGA prefers not to refer you to those “cures”.  However, milk has been studied as a fungicide and found to be effective for powdery mildew⁷.  We recommend that you do some careful online searching before trying other home remedies.

Over recent years, several commercial products have become available to treat plant disease organically based on various salts and other ingredients which have not been disclosed by the manufacturer.  However, the majority of commercial fungicides are based on artificial chemicals and many have a number of unwanted side effects on wildlife and pets.  We suggest that you look at SGA’s app WiseGardening: Choices to Protect You and the Planet before making any purchase.

References

1. Douglas SM.The Connecticut Agricultural Experiment Station. Plant Health Problems.
2. Velasquez AC, Castroverde CDM, He SY. 2018. Plant and pathogen warfare under changing climate conditions. Curr Biol. 28(10): R619–R634.
3. Jain A, Sarsaiya S, Wu Q,  Lu Y, Shi   2019. A review of plant leaf fungal diseases and its environment speciation. Bioengineered. 10, 409–424.
4. Ausveg.Overview: Pests, diseases and disorders.
5. Williams SD, Boehm MJ, Lopez-Nicora H, Department of Plant Pathology. Nematode Diseases of Plants
6. University of California Agricultural & Natural Resources. How to Manage Pests in Gardens and Landscapes.
7. Ferrandino FJ, Smith VL The effect of milk-based foliar sprays on yield components of field pumpkins with powdery mildew. Crop Protection 26, 657-663.