Botrytis Control

Botrytis (Botrytis cinerea), commonly known as Grey Mould, is a common and sometimes serious problem on horticultural crops. It is especially severe under cool and wet conditions or where crop management has allowed the pathogen to take hold. Botrytis c. has a wide host range and can affect most crops. 

Botrytis is a very weak pathogen and usually acts on the plant as a secondary disease, in that it infects through a wound or weak area on the plant. After a short time the disease can produce masses of grey spores called conidia that are air borne and dispersed by air currents and over head irrigation sprays. It can also be spread on tools and clothing. You need to act as soon as the first signs are spotted, although it is better to act on incoming weather conditions before the disease starts. 

Botrytis spores can also cause trouble if they start to germinate on petals and in buds (see below). Botrytis is part of natures waste disposal system and is only there to break down dead and dying material. Botrytis (nor any other pest or disease) does not attack healthy plant tissue. In the growing situation Botrytis is a problem on weak and unhealthy plants, on cut stems, on flowers in houses with a high relative humidity (RH), on dense crops that have a high RH in the growing canopy, on dying flower buds and in houses with dirty glass or plastic, blocking out UV light, which can kill the botrytis spores in the way nature intended and help dry the leaves  from excess water                           .                                                                                                             

Botrytis along with other harmful pathogens start after what is known as degree hours which basically is the time the leaf surface remains wet. The simple rule is that the longer the leaves remain wet and the warmer it is the quicker fungal diseases will start so the secret is to make sure the crop remains as dry as possible at all times and you have products on the leaf that are a barrier to infection.

It is also worth noting that the shadier the area the worse the problem as it is with all pathogens. As you enter wet times of the year it is vital to allow as much light into the house as possible so make sure covers and glass are clean and in the case of plastic make sure they are replaced on time. Plastic covers are referred to as 5 year (or 3 year etc.) because that is the amount of time they will allow an acceptable amount of light to pass into the growing area; after that even though they are still strong the light levels drop and the incidence of disease increases so they need replacing and the cost is offset by the improvement in crop quantity and quality. In many parts of the world Luminance polythene is available which cuts out 15% of radiated heat in the summer and in the winter it diffuses the light giving a greater light level at ground level so this may be worth looking at.   

Crop Management

One of the most efficient control methods is air movement as the spores simply cannot land on the crop and moisture is dissipated, so fans and good air movement is essential 24 hours a day when high RH conditions prevail 

(It is vital not to use strong airflow in a house that has Powdery or Downy mildew as you will spread the spores). It is important to make sure that the air flow moves around the house in a smooth clean fashion and if the fans are correctly placed the spores will be blown out of the house when the vents are opened. The rule of thumb is that you should always feel a draught everywhere within the glasshouse.

Make sure that the fans are spaced correctly in the house to get a good flow of air without ‘dead’ spots where disease can break out and then spread. Start at one end of the house with fan number one looking up the house and placed roughly a third of the way up. Fan number 2 (if in the next bay) should be facing the opposite way also a third of the way in. This gives excellent up and down air movement whilst the houses are shut. Please note that in single sheet plastic houses strong airflow will increase the amount of heat needed as there is increased heat exchange through the plastic so good insulation is needed and the down side to this in heated houses is that fuel costs will increase.

It is important to remember that often, Botrytis is encouraged and made worse when a crop is cleaned or picked. Workers pick up an infected plant and then carry those spores to the rest of the crop. Only clean when absolutely necessary and then spray before and after the process. Spores can remain dormant for 10-12 weeks so good cleaning is essential which is where E-Clean^TM really comes into the picture (See Nursery Hygiene). Germination is triggered in warm damp conditions by low light, plant stress or increase in crop load (more stress).

Hygiene

Hygiene is also vital as this keeps the number of free spores down to a minimum and using the right materials can kill botrytis spores on the plants and surrounding benches.  Botrytis overwinters as mycelium and or sclerotia in the growing media so a good winter wash of the plants is well worth while. Botrytis can live on host weeds and plant debris for several months or even years so a weed free growing zone is well worth the effort plus it is good horticultural practice. Constantly remove any dead plant material from the house and make sure paths and rows are treated with E-Clean^TM to kill spores, which reduces the problem from the start. When removing dead material from the house it is wise to spray the material before lifting it to reduce the spreading of spores and after the work is finished to kill any remaining spores. 

For growers in the soil another option to continually cleaning out old and dead material is to leave the fallen leaves on the ground and regularly drench the area with a Trichoderma/Bacillus product like FKS-Tricho-Sheild^TM & FKS-B-Sub^TM. This will encourage the rotting process of the leaves into the organic humic layer in the soil whilst at the same time making sure harmful pathogens do not build up. This will encourage new and healthy root but you must remember that if you spray or drench any agro-chemical you will damage this layer and allow disease to set in. You will also need to increase the Carbon quantity in the soil as a feed for the natural life, but as this also increases the efficiency of the N it is well worth while, for example use Kelp-372^TM

This double job will pay for itself very easily in saved crop.  Many growers add E-Clean^TM to their spray to increase efficacy although this would have to be tested on a small amount of crop to start with. This is just as important in the pack house and store to improve vase life. Clean sharp secateurs and knives also help. Note that if beneficial bacteria and fungi are used as part of your pathogen control program you should not use E-Clean^TM or similar on the foliage as it will have a detrimental effect.

Dead and dying material in the house will give a food source to many harmful pathogens and pests, all of which will damage your crop and profit margin. Would you eat your dinner off your benches? This is as important as the phrase ‘would you drink your irrigation water’? Remember that most of the pest and disease problems that occur around the nursery, originated in your pipe work, your benches and your floors. By formulating a hygiene policy using E- Clean^TM and good growing practices, that we can provide advice on, you can have a perfectly clean surface on which to grow your crops and consequentially drastically reduce problems when the weather turns against you. 

Weed Control

Weed Control is important also, because the extra RH that is found in a dense weed area (due to poor air movement) helps botrytis germinate. When botrytis germinates in a wet area (especially on a petal) you can get ghost spotting. This is caused by the oxalic acid in the spore burning the petal. The damage then allows the botrytis to enter the plant as a secondary disease. In some crops such as Roses, even a clean crop is at danger because of the density of the crop so special care must be taken to keep the open petals dry.

Obviously the irrigation of the crop is vital to get right so make sure you water only when the crop/soil needs it and make sure no moisture is added to the house after mid day so there is time for the house to dry. This also means all spraying needs to be early in the day to allow complete drying before the evening. 

The secret is to be aware of when the RH is high, leading to a dew point on the leaves. This can also occur early in the morning as the sun rises as this leads to rapid air temperature rise. This means that the air is warm and the plants are [relatively] cold leading to a dew point on the plant which quickly leads to botrytis spotting. This is an essential time to have the fans on and ventilation open. 

One thing worth trying is to cover the floor with weed matting which allows easy cleaning, no weeds and reduced RH as it will be dry on top as you go into the evening. White weed matting gives the added advantage of increasing light levels in the bottom of the crop which is proven to reduce harmful pathogens and increase plant health lower down. 

Another point to remember is that a lot of the leaves falling from the plant are caused because the plant has too much Nitrogen and or Manganese causing the plant to drop the older leaves, so watch the nutrient levels. If you feed correctly at the right times of year the leaves will not drop leaving them working for the plant. Healthy leaves produce more sugar for the plant and therefore better protection against all disease. 

Again growers in the soil can gain from this, as 30% of all the sugars produced by the leaves are put into the root zone to feed the natural flora and fauna (beneficial Bacteria and fungi) and if this is done well you will save a fortune on nutrients. One reason for encouraging the beneficial Bacteria and Fungi is because there is 740,000 tons of Nitrogen floating above every Ha of land and if the right bacteria are encouraged your plants will be able to use this for free, rather than you having to buy expensive fertilisers.  

It is important to watch your crop both at sunrise and sunset when there can be a large temperature fluctuation. In the morning as the sun rises the air temperature quickly shoots up leaving the plants comparatively cold, which causes all sorts of problems as we reach a cold dew point on the leaf. Good ventilation at sunrise can stop many of these problems as can making sure the crop and house is as dry as possible well before sun set. It is well worth considering having the wall of the house facing the sunrise shaded all year round to slow down the temperature rise first thing in the morning.

For problems caused by prolonged periods of wet damp weather the only options are keep the area as clean as possible, vent as much as possible, irrigate only when very necessary (don’t forget the soil/growing media will be holding a lot of moisture and nutrients that the plants can use) make sure the plants have been ‘hardened by correct feeding (high P/low N) and foliar spraying in the run up to the wet periods. 

Also regular sprays with E-Clean^TM will make sure that the number of spores available is kept to a minimum whether on the plant or surrounding surfaces. Always make sure that when you use a ‘leaf cleaner’ such as  E-Clean^TM you must also add or spray with products that give the leaf protection such as E-Boost^TM or Trichoderma/Bacillus based items (FKS Tricho-Shield^TM/FKS B. Sub^TM) that give a physical barrier to any landing spores. 

For those growers who are suffering from Botrytis post harvest you need to examine where the botrytis is being activated. The first point on this to remember is that the spores that cause the problem in the vase come from the growing area and the possibly the packing area, so it is vital to treat the plants in the house regularly and examine what you do in the pack house. Is the concrete floor painted, is the area sterilised on a daily basis with a  E-Clean^TM ,spray or mist through a ULV machine in the coolstore with E-Clean^TM can be very effective. Have you done a vase test to see if flowers going through the chiller suffer more due to the temperature changes than flowers taken straight from the greenhouse? The more things you try, the more you will learn and the easier it will be, to make good decisions. 

At Franko Solutions where we do many of our trials we use  E-Clean^TM in the flower buckets as a preservative, as tests have shown that at 1ml/lt (0.1%) we get better protection from botrytis than actual specialist flower preservatives, plus we only change the water once a week instead of every 3-4 days as is usual. Also we have tested a mix of  E-Clean^TM /E-Guard^TM/ Meltdown^TM  as a flower dip and found that plants under botrytis attack last considerably longer than flowers not dipped plus it kills any thrips that may be hiding in the bud. Contact me if you are interested in discussing this further.  

The Natural Way

There are now several environmentally safe methods of controlling botrytis and keeping the spores to a minimum. I have already mentioned the E-Clean^TM that can be added to chemicals as a wetter and used as a hygiene product. We now have a range of products that can help reduce harmful pathogens by directly and indirectly. We have been having excellent results with Thor 413^TM which is a readily available source of silica that hardens the cell walls which has good results against Botrytis, powdery mildew and Downy mildew and could be even better if mixed with a plant growth promotant such as E-Boost^TM (which also helps the plant fight off diseases naturally), ProAlexin^TM (derived from citrus fruit) and a micro nutrient mix where necessary to strengthen the cell walls. The Thor 413^TM/E-Boost^TM/ProAlexin mix is popular amongst growers as there is an immediate positive response in growth (don’t forget to test a small area first on your crop.

In duller weather Goldrush^TM is added which helps the plant form sugars in times of stress helping keep crops frost protected and in full production. Again you should test a small area first to make sure there are no issues with the varieties that you grow.  

Growing Media

As with all diseases the healthier the crop the less you will suffer so a very healthy growing media containing natural flora and fauna is the best investment that you can make against disease of any sort. This means that we can beat many harmful pathogens in an indirect method. For those growing in the soil you have the advantage because the soil is a superb buffer and it has been growing long enough to know what it is doing. The down side to this is that when mono-cropping the plants always take the same nutrients and allow the same pathogens to build up. To counteract this we have the organic materials to fight in the soil on our behalf. The addition of Trichoderma 

(FKS Trichoshield^TM)and Bacillus subtilus (FKS B. Sub^TM) as mentioned will give you an immense army of workers that busily munch away on the harmful pathogens that will attack your roots. 

Trichoderma (harzianum and Koningii) plus Bacillus subtillus form a superb team to live around and on the root giving instant protection against root diseases by competing with the harmful pathogens for food and space and many people are finding that when this mix is sprayed on most harmful pathogens are unable to get into the leaves. Remember if using as a spray on leaves it is vital to keep spray pressure below 70psi and to add a UV stabiliser such as Sentry Oil^TM otherwise the products will die before being able to work.

In other crops Bacillus subtilus is widely used to stop many known problems such as club root and downy mildew. These natural pathogens thrive in well maintained soil working around and on the roots. Remember that 30% of all the sugars that the plant produces are exudated into the soil out of the root. This is 

not done by accident, rather it is done to feed the natural flora and fauna that the plant expects to find there, as an essential part of healthy growth, as the microbes break down the nutrients into a form that the plant can absorb. This is also true of course of plants grown in all other growing medias so whether you grow roses or cabbages it is vital to support the soil food web which will in turn look after your crop and help the roots absorb difficult nutrients such as phosphorous and sulphur. 

To assist these fungi and bacteria we add products such as Terra CMB^TM and Terra Liberate^TM along with liquid Kelp 372^TM to help nourish them whilst at the same time helping the plants form new roots giving stronger plants which of course leads to less disease. Another good help is to irrigate the crop into the soil with Transplant 122^TM which quickly helps the crop get over the shock of planting and rapidly promotes new root growth. A healthy rapid growing root can easily live with the normal amounts of root disease that is in every soil and if the roots are working they will help the plant fight off airborne pathogens such as botrytis.

Of course, one of the main ways to keep botrytis and other diseases at bay is to feed the crop correctly and to change the feed as crop stage and time of year changes. By giving a Nitrogen rich mix on a year round basis just encourages soft weak growth that actively encourages pest and disease. 

Get to know what your crops need and react to how they are looking. Remember to change the balance of the feed by reducing N and increasing K as winter approaches to strengthen cell walls helping the plants fight the cold and keep out disease. I have recently discovered that varying the nutrient balance actually stimulates active growth along with plain water cycles once a week to keep EC levels down around the root. I find that restricting the EC and adding feed through the leaves gives a rapid and worthy response and helps strengthen the plant against fungal attack. I can, of course, discuss this on a one to one basis on the Franko Solutions email to help you improve production. 

Also as the amount of water a crop wants goes down in the winter remember to increase the feed rate so the plant can continue to actively grow when the chance is there but alter the balance to restrict NO₃ input. I often see crops growing outside that are assumed to have a disease when all that is really wrong is the crop is hungry, and this of course can lead to problems in the long run.  

Chemical Control

The most common chemical protectant is one of the chlorothalonil products that protect against a wide range of diseases although there is a lot of resistance in this chemical group. Chemicals from this group are favoured as a preventative which leads to the problem that when you do get an outbreak, the surviving spores seem harder to kill.

Iprodione is the only true curative product we have left but remember that once you have sprayed twice the chemical then starts to build up resistance so only keep as a last resort. The strange thing is that when you do get resistance, if your stop using for two years the resistance actually goes completely and you can use it again whereas with other chemicals the resistant pathogens remain on your site for a long time. Thiram, carbendazim and prochloraz can be used although as with iprodine they are best kept in reserve if a problem breaks out because they are so strong that you run the risk of forming a totally resistant spore. These are only a few of the available range of chemicals and the important thing to remember is to always use a chemical from different families to avoid resistance building up. It is important to apply in the right amounts, at the right pH (many chemicals simply won’t work at the wrong pH) and with a good sticker. Even without overhead irrigation or rain many fungicides simply fade away very quickly.

Nick Woodyatt

Research and New Product Manager