Prevention and control of Downy Mildew Disease

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Plant disease: Downy mildew

Downy mildew is a disease that can affect from seedlings to mature plants, it is caused by afungus-like organism called an Oomycete.The organism penetrates the tissues under wet conditions and grows out causing off white, fuzzy patches under the leaves and yellow discolouration on the top. These infections then release spores and spread to other brassicas by wind and rain.


The primary symptom is a white to blue or purple, downy growth, usually on the undersides of leaves and along stems, which turns black with age. Pale green to yellow or brown areas usually develop on the upper leaf surface opposite the downy growth. Other common symptoms for downy mildew include mottling or spots on the leaves. Plants that are affected by downy mildew, may be stunted or have an unnatural amount of leaves falling off.

Plant type



The entire plant becomes covered in a whitish coating of the pathogen and dies rapidly.

Lager, mature plants

Yellowish patches of discolouration on the upper surfaces of leaves. On the corresponding lower side should be a fuzzy, hairy whitish outgrowth. Eventually, the affected tissues die, shrivel and fall off.

Cauliflower and radishes

Leaf damage may be accompanied by internal browning of the cauliflower curds and radish flesh.


Pods will usually be covered completely with the growth and leaves distorted.

Cause and transition: mildew actively releases spores that are then spread by wind, rain, and in seeds. Water that sits on the leaves of the plant gives the downy mildew a way to infect and spread on the plant. The spore of downy mildews spreads by swimming through water until they come across live plant material to infect. If the hosting plant tissue dies, the pathogen also dies, or sometimes forms a dormant resting spore. The timing for downy mildew is late spring in younger plants, especially in greenhouses and early autumn in maturing crops outdoors.

Conditions that aid the development of mildew:

  • Cool, moist conditions
  • Host weeds
  • Crop residues
  • Poor aeration
  • Overcrowding plants

Older methods:

Downy mildew was prevented and controlled by watering your plants from below, rotating the plants annually and removing any infected plants. Gardeners had to be careful not to overwater or overcrowd plants and fix any poorly drained soil. Other avoidance strategies included: eliminating weeds and maintaining balanced soil fertility.

Newer methods: sprays that include bicarbonate are now available from gardening stores however fungicides, in most cases will not help as downy mildew is not exactly a fungus. Applying maneb, copper or zineb is also quite effective against downy mildew but the residue on the plants and vegetables need to be taken into consideration before consumption. Due to developments in science and technology, buying disease-free seeds or a more resistant variety of a plant has become possible. This may cost more initially however it can save a lot of time, effort and money in future.



Use resistant varieties of plants where possible to help reduce chances of infection.

Seedbeds should have well-drained soils and be situated away from hedges and bigger plants.

Use only certified diseased-free seeds for sowing. Transplant only healthy seedlings.

Seedlings should not be excessively watered.

Ensure proper land preparation to make sure that your soil is well drained.

Weeds should be eradicated in and near seedbeds and out in the production fields.

Provide adequate plant spacing to reduce the density of the canopy and minimise humidity.

Crop residues should be removed from the field after harvest.

Pruning of new growth also helps proper plant's aeration.

Avoid sprinkler irrigation because it creates a constant moist environment.

Remove infected plants and prune infected shoots.

Thin plants to reduce plant density and increase air movement.

Properly dispose of collected diseased-parts either by burning or burying them.

Time irrigations so that they do not elongate leaf wetness.

Avoid overhead watering. It lengthens the duration of leaf wetness and favours further development of the disease.

Alter planting dates to avoid periods of high disease pressure.

Plough-under all the plant debris after harvest.

Fungicides on infected plants may not kill the infection but could weaken the disease.

Practicecrop rotation annually to reduce chances of infection.

Put maneb, copper or zineb on plants as prevention and on the infection as control.

Advantages and disadvantages:




Older approach

  • Safe and natural: no chemical on humans and plants.
  • Effective, even though the older methods are quite primitive, removing the infected plants works. The disease may reappear however if the preventative strategies are kept up then there should be no problem.
  • Requires a lot of time, the gardening will be a full time job if the plants are to be disease free and always in perfect condition.
  • Much strength and effort is needed to move plants around and removing diseased weeds and plants.

Newer approach

  • Buying a disease free plant or applying chemical treatments is less time consuming than the older approaches.
  • It is also less strenuous in terms of not having to pull weeds and move plants around constantly.
  • Fungicides or other preventive sprays may not work as they claim.
  • Buying disease free seeds and resistant varieties does not mean that those plants cannot contract the infection.


The successes of both the older method and the newer method have limitations in terms of time, effort, costs and effectiveness. The older method is effective to a certain extent but is extremely strenuous and time consuming. On the other hand the newer method costs more and is not as effective but people adopt it because they are generally too busy to take proper care of their garden.


If plants are infected with downy mildew, the older control method of downy mildew is the most effective. The reason is that once a plant has been infected with downy mildew, there are no effective chemical controls. However incorporating some preventative chemicals and using resistant varieties from the newer method will give the best possible outcome. your plants have downy mildew, the best thing you can do is to try to eliminate moisture and humidity around the plants. As mentioned, make sure your are watering from below. If possible, try to improve air circulation through selective pruning. In enclosed environments, like in the house or in a greenhouse, reducing the humidity will help as well. Gardeners need to apply both strategies to lessen the chances of infection or to bring downy mildew under control. Further research needs to be done to develop an actual treatment for downy mildew because there is no effective organic or chemical method as of yet.

Animal disease: Tasmanian devil facial tumour disease

The Tasmanian devil, due to an infectious cancer called devil facial tumour disease (DFTD), has experienced a severe population decline in recent years. Infectious DFTD cells first appeared in the body cells of a healthy Tasmanian devil in in the 1990’s. The tumour cells can only survive within Tasmanian devils and not even the devil’s closest relative: the quolls. Their immune system would recognise the tumour cells as a foreign object (antigen) and destroy them. However, like many other cancer cells, DFTD developed mechanisms to avoid being killed by the devil’s immune system.


Once a devil is infected with DFTD, the disease starts to appear within a couple of months, as tiny lesions or pimple-like lumps. These small lumps quickly turn into large a tumour that distorts and dements the face area. Soon after it will be difficult for the devil to eat and drink, leading to death by hunger, dehydration and the breakdown of body functions. In the later stages, the cancer may reappear in vital organs, like the lungs or the brain.

Cause and transition:

Recent research has found the reason that DFTD cells are not rejected is because they do not have ‘major histocompatibility’ (MHC) antigens on the cell surface. These are immune recognition molecules and without them, the cells are undetectable to the devil’s immune system. This allows the tumour cells to be passed on to a healthy devil undetected; making it transmittable even though it should’ve been labelled as an antigen and destroyed. The lack of genetic diversity in Tasmanian devils has only made it easier for the tumour to spread and studies suggests that the tumour is still evolving.

graph of decline of tas devil

DFTD spreads when the Tasmanian devil bites or scratches each other, transmitting the cancerous cells that grow into enormous tumours. Passing on tumour cells generally occurs during fights associated with feeding, territoriality and mating. In addition, it is also possible that cannibalisation of deceased devils, as well as sexual encounters may allow the disease to spread. As a result the population of the devils has been on a steady decline as seen in the graph to the left from

Older method:

The Tasmanian devil’s extinction was prevented by keeping them safe in captivity, isolated from infected devils. Surgery and chemotherapy was trailed and was successful to a certain extent but proved to be difficult to implement on a large scale. A large insurance population of healthy Tasmanian devils was also bred to keep them away from the disease-front.

Newer method:

There are many programs that pursue vaccine development with the aim of protecting the healthy devils and repopulating the devils that have resistance to DFTD. Further research has found that the molecule called EBC-46 can slow down the rate of progression of DFTD. EBC-46 disrupts the tumour blood supply and destroys tumour cells while leaving the surrounding healthy tissue intact. While EBC-46 is not a good solution for the control and management of DFTD in the wild, it has potential to slow the rate of growth of the tumours in captive devils.

Advantages and disadvantages:




Older approach

  • Isolating a population is effective in ensuring that the Tasmanian Devil will not become extinct in captivity.
  • This also allows researchers more time to look for more efficient preventative measure or possibly a cure.
  • Surgery and chemotherapy were fairly successful in the tumour’s early stages.
  • Requires a lot of time, funding and effort to find devils and capture them and breed them.
  • The ones that are bred have to remain in captivity otherwise they can catch DFTD from a wild devil.
  • Even though surgery and chemotherapy works to a certain extent, it can only be applied to a few individuals and not the whole population.

Newer approach

  • Once the vaccine is produced it will be a good preventive or control method for captive devils.
  • Trials have been effective in eliminating only the cancer cells and not destroying the healthy tissue around it.
  • Effect of EBC-46 is still not fully understood so further research must be done before it can be applied in a vaccine.
  • Cannot prevent or treat wild population due to difficulty locating, catching and giving the vaccine.


The older method has been effective in keeping the Devil population from completely disappearing however it requires a lot of resources. For example to save one devil, it needs to be located by a team of people, then captured, then transported to a clinic for treatment. If it’s in the early stages of DFTD like the photo on the left, the vet may perform surgery and chemotherapy. Even then, the chance of survival for this one devil is slim and the resources that are expended don’t meet the results. If the devil found looks like the photo on the right, then it is beyond saving because the cancer has progressed too far and spread throughout its body. The other old method of isolating and breeding healthy devils is much more efficient and gives more time for the research of a prevention or cure.

The newer method is a vaccine that incorporates EBC-46 and has been trialled as effective but more research is still underway before it can be produced into a vaccine. This idea is a lot more efficient than the older methods because it uses less time and effort to be applied to a bigger population.

6 month old Tasmanian devil joeydevil facial tumour disease


The problem is that the cancer cells are ‘invisible’ to the immune system but the chemical EBC-46 is able to destroy the tumour cells, leading to the possibility of a vaccine that could prevent or slow down the rate of DFTD. With further research, it is only a matter of time for the scientist to understand how EBC-46 works and how to incorporate it into a vaccine. To maintain a healthy devil population during this time, the older methods of trying to save the infected devils in the early stage of the cancer and keeping a populated isolated in captivity help to keep DFTD at bay. Using both methods is a good way to achieve the best possible outcome of keeping the devils alive and finding a prevention or cure.