By Ruth Vaughan
This is a story that is told a lot! However there is more and more pressure to reduce our reliance on pesticides! There is increasing pressure to limit pesticide residues in roses to less than 10 active ingredients and less 70% of the MRL’s of vegetables. Yes – we don’t eat roses, but they are considered a luxury item as opposed to an essential item – hence this pressure to limit chemical applications is more geared around environmental concerns.
Diseases in roses limit production and quality. The most common diseases are fungal diseases. For them to start growing you need a combination of factors to come together at one point in time:-
1) Starter material – generally spores or spore carriers (condia / sclerotia)
2) Something for the fungus to feed on
3) Entry points into the plant
4) The perfect climatic combination of temperature and humidity
5) Varietal susceptibility
Removing any of the above parts of the equation will stop your fungal infection from starting. You cannot have a fungal infection in the absence of the spores. Start clean and carry on clean is the best advice. This is not always possible, and the days of toxic soil sterilization are over. However, to reduce the risk of infection always make sure you source your rose plants from a reputable propagator and scout the plants at arrival.
Discard any plants that have insects on them or disease symptoms. You would be advised to do a pathology scan on the soil before you plant. If pathogens are picked up -solarize the soil / or plant a radish break crop. There are many biological agents that are commercially available in Kenya that work with the plant to improve disease resistance or prevent infection. These have no active ingredients and can be very effective. The old adage prevention is better than cure is very true!
Good crop husbandry is essential and removing open flowers, dead leaves and stem parts from the greenhouse is a must. Very often the decaying plant tissue will provide the start material for the next infection. Spores can be moved around in wind and water. Limit wind movement in infected crops and make sure your greenhouses, gutters and drip lines are properly maintained.
Leaking rooves and gutters and spraying drip lines are prime disease spreaders. Keep equipment clean – secateurs / jembe’s / spray hoses / hose pipes / flower buckets – all should be dedicated to certain areas and sterilized regularly. PPE can also move spores around and again should be clean. Boots and tires move around quite a bit of soil, which can contain spores and nematodes. Sterile foot baths into each house are a good idea.
Tissue to feed on
Fungi need food. They get this from dead plant tissue (decomposition) or living tissue (disease) or both. Plants that are weak have much less resistance to diseases. Overfeeding nitrogen creates very soft roses that are prone to disease. If all the nutrients in the plant are in balance – not only will you get very good production and quality but your plants will be less prone to disease. A comparative leaf analysis of the top (young) and bottom (old) leaves will pick up nutrient deficiencies of both plant mobile and immobile nutrients. Deficiencies can then be rectified by foliar feeds and changing the fertigation recipe.
Nematodes also encourage weak disease prone growth. Keep your nematodes to a minimum. Proper soil moisture levels and aeration are required for optimum nutrient update into the plant. Saturated soils contain not oxygen – so nutrients that require energy to be taken up by the plant cannot be taken up, iron is a prime example. Very dry soils or soils with a high EC are also a no-no, because plant nutrients that enter the roots via the `transpiration stream’ cannot be taken in.
Entry points into the plant
Many diseases need an entry point or wound to be infectious. When planting young plants keep root and plant damage to a minimum, and plant with Trichoderma for extra protection. Other wounds are caused by soil scraping, wedding, crop thinning, and hoses (spray and humidity). Hose damage can be prevented by putting posts at the ends of the beds. Nematode and insect damage are also good entry points for diseases. So keep your nematodes and insects to a minimum.
The one wound we cannot prevent is harvesting wounds – but damage can be minimized by cutting close to the node and ensuring that the secateurs are very sharp and clean and havethe correct spacing for the stem thickness on the ‘holder’. Cushing the stems during harvesting not only encourages diseases but provides a safe place for mealy bugs to breed. Harvesting late in the day in damp weather will leave the cut point wet overnight.
Many fungal spores need a particular set on climatic conditions in order to germinate, grow and penetrate the plant. It is worth noting the particular conditions required for the diseases that plague your roses and try and reduce them. One should measure the temperatures and humidity at plant level in the greenhouses. Climatic control is most important for disease prevention. Weather stations and predictive disease modelling are in development – these will be able to alert you when conditions are rife for a particular disease so that you can do preventative sprays.
Side curtains, fans, movable rooves, very high rooves are all tools that can be used to manipulate the climate in the greenhouse. Soil moisture also plays a major role in determining humidity. A soil moisture probe will allow fact based wateringof plants to prevent saturated soils, dry wilting plants, day time drought and high night time humidity – all disease drivers.
There is a big variation in disease susceptibility between different varieties. If you live in a downy area don’t plant a downy susceptible variety. Always consult your breeder when selecting plants.
Common Rose Plant Diseases
The following three diseases that account for over 90% of fungal spray. We are all familiar with them!
Botrytis – Botrytis cinerea
Grey mold appears on dead tissue as a grey brown dust making fungal fluff. It creates a big problem in postharvest as one small lesion is sufficient to make a whole flower rot away. It affect flower quality / market access and flower prices.
Botrytis is known as a weakness disease – infecting living tissue only under certain favorable conditions; excessive humidity, high crop density, cold, excessive nitrogen application and low light conditions. Most infections on living tissue take place via wounds except for on the flower, which is very tender, Millions of botrytis spores on dead rose tissue – greenhouse cleanliness is very important although very often heavy botrytis infections in flowers are associated with thrips damage – so it’s good to assess and treat the problem together.
Spores can breed up in millions in dead plant tissue on the soil surface and very easily move around the greenhouse in air currents or water droplets, removing dead tissue and a preventative spray after a greenhouse cleanup (which dislodges and spreads spores) are recommended.
Spores are always present in greenhouses – so proper prevention is via climatic control or preventative sprays. At 29C spores require a humidity of 90% for 6-8 hours in order to germinate and penetrate flower tissue. (They need a humidity of >95% for 20 hours to penetrate leaf tissue). Reducing the humidity below 90% via heating, ventilating and judicious planning of sprays and watering will greatly reduce the risk of botrytis. Keep the crop open, and balance the flush for better air movement inside the crop. Low light levels can greatly increase risk of infection.
Powdery mildew – Spaerotheca pannosa
Can occur all year round. A white powdery fungal fluff develops on leaves – consisting of mycelium and spores carriers. The fungus develops outside the plant tissue and can be rubbed off. If can also affect stems and flower buds. In young tissue it cause deformation, as it kills the growing cells, in older leaves brown spots develop. In severe infections it can be seen on the underside of the leaves.
Susceptibility can be very varietal, while yellow varieties tend to be the worst hit. Weak plants that have a nutrient deficiency are also very susceptible – and often proper plant nutrition can prevent infection. The infection generally starts in one spot in the greenhouse and very quickly spreads. The spores are very light and can move a long way in gentle air currents.
Spot spraying the infected area as soon as possible and limiting air currents to a minimum. Preventative spraying on the rest of the crop is advised. Spores contain 70% moisture and do not require water for germination, however a high humidity will enhance germination. Moisture droplets inhibit germination – and can be used as a control measure -however this will weaken the plant tissue making consequent infection much easier.
Once the spores have germinated they need a short time at >21 C and a humidity of 90-99% in order to infect the plant. Spores are generally released at midday. Spores do not live long and die within 2-3 days if they do not germinate. Very hot dry days and cold wet nights increase the risk of powdery mildew substantially.
Downy mildew – Pseudoperonospora sparsa
The rose grower’s nightmare, only flourishes under humid conditions, but can defoliate a whole greenhouse overnight. Again variety and plant health play an important role. Optimal watering, feeding, keeping the crop open and growing out of flush are important. Infection is generally limited to young plant parts. Symptoms may be seen on leaves, stems, petioles, petals and sepals.
The first symptoms on the leave soften appear as oil spots, yellowy areas with a red edge that can be mistaken for pesticide scorch or nutrient deficiency. Infected leaves have purple red or grey black spots, and a fungal fluff develops on the underside of the leaf. The leaf will drop soon after the fungal fluff develops. The spores require a thin layer of water (condensation) in order to germinate and penetrate the plant, which is generally through the stomata. A humidity of <85% considerably reduces the chance of infection. Spores do not develop <5C and over 27C. (In fact at temperatures over 27C for >24 hours the spores die.)
Downy mildew attacks are most prevalent when the weather changes – this may be due to over or under watering the plants when there is a rapid change in plant water requirements. A soil moisture probe will give you the tools to adjust watering rapidly.
Large temperature fluctuations -which cause condensation – should be avoided. High greenhouse rooves in downy prone areas can greatly reduce temperature fluctuations, condensation and the risk of downy and botrytis. Downy mildew generally produces its spores at night – so prolonged leaf wetness should be avoided.
Control of downy is via preventative spraying. The mycelium grows inside the plant tissue. Infected plant tissue cannot heal and should be cut out of the crop and removed. In addition to the spores that form under the leaves that are responsible for direct dispersion of the disease during infection, downy mildew produces thick walled resting spores inside plant tissue. These can persist for a very long time, and will be released when the surrounding plant tissue decomposes. It is therefore very important to cut out, and remove infected stems. Do not use infected tissue for compost. It should be burnt.
For more information on disease modelling or plant nutrition – please contact firstname.lastname@example.org
Ruth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh Journal, HortiNews and Floriculture. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. Loves visiting farmers and seeing all the different farming methods