Plant nutrition a solution against botrytis
BY Ruth Anita Vaughan, Technical Director Cropnuts
Botrytis, commonly known as grey mould is one of the most damaging diseases of cultivated roses worldwide. It also causes untold damage in a wide range of bulbous, flower, fruit, vegetable, fibre and oilseed crops. It affects crops not only in the field and greenhouses but also during storage and shipment. Botrytis is necrotrophic, a parasitic organism that injects toxins into the living cells of its host and then feeds of the dead matter. Creating a slimy, smelly mess. Fungicides are widely used to suppress botrytis, however due to the genetic plasticity of this fungus, fungicide resistance and other factors compromise the efficacy of the fungicides.
The disease triangle
For botrytis to infect plant matter you require the spores (the pathogen), a susceptible host and a favourable environment. (Refer to the disease triangle). Botrytis feeds on living and dead matter and produces abundant conidia (spores) that are moved around in water splashes and wind current. We can safely assume, that in field conditions, botrytis spores are everywhere. For a favourable environment, the spores need free water on plant surfaces to germinate, and high humidity and cool mild temperatures to grow fast. Cultural techniques that suppress botrytis include sanitation, ventilation, reduction of humidity, and watering techniques that reduce the time the plants are wet.
For botrytis to infect plants it needs susceptible host tissue. Botrytis cannot infect healthy plant tissue. It needs an entry point. This would be damaged cells, (thrip damage is a very common entry point), wilted cells, stressed plants, burst cells, or senescent cells. Once the botrytis has got a foothold into the plant it can spread quickly.
A healthy balanced soil and a balanced fertiliser and watering program will ensure healthy vigorous plants that can resist diseases. Healthy vigorous plants are faster growing with a shorter crop cycle, stronger with lower disease pressure, higher yielding, better quality, longer vase life and better opening in the vase.
How plants resist botrytis
Initial infection is prevented by mechanical barriers, a thick waxy cuticle and strong cell walls. Plants produce natural defence compounds, and compounds that can de-activate the toxins produced by botrytis. They can repair cell damage with silicon compounds and calcium spiking around wounds.
What makes plant cells strong?
Strong turgid plant cells are the key to resisting botrytis attack. Consider a plant cell is like a balloon filled with water in a cardboard box. The cardboard box is the plant cell wall, the balloon is the cell membrane and the water the contents of the cell. When the cell is turgid the membrane is touching the cell wall and the cell has its maximum strength. If you remove water from the balloon, the cell membrane shrinks away from the cell wall, (wilting) and the cell becomes weak. If you overfill the balloon, the cell will burst, creating a wound and entry point for botrytis.
The turgidity of cells is dependant on good soil and root health, a large root volume and optimum moisture and oxygen levels in the root zone to ensure a good supply of water and nutrients to the plant. Too little or too much moisture in the root zone will cause the plants to wilt. High soil salinity increases osmotic stress and causes plants to wilt. High sodium in the soil causes cells to burst. A healthy xylem and phloem are needed to transport water, energy (sucrose) and nutrients throughout the plant.
Important plant nutrients and what they do
Calcium, as always is King in Botrytis prevention. Calcium is the building block for calcium pectate – which determines the cell wall strength. Low plant calcium results in weak cell walls and cell burst. Calcium levels have a huge impact on shelf life and quality. Calcium uptake is reduced in cold wet humid weather, if calcium levels are low in the soil, in low pH soils and if too much potassium, magnesium or ammonium is applied to the soils. Calcium cannot be translocated around the plant. The calcium levels in the plant sap at the time that the cells divide determines the final cell wall strength. In cold, wet weather it is a good idea to boost calcium levels with regular foliar sprays. Boron is important for calcium and silica uptake; however, boron can quickly become toxic and cause cell burst. The levels of calcium and boron and the calcium boron ratio are important.
Potassium is responsible for osmo-regulation and controls the opening and closing of stomata. Low sap potassium results in premature wilting and hot plants. Potassium facilitates starch and protein synthesis and determines flower head sizes. A high potassium to low calcium ratio produces large headed, weak flowers that are very susceptible to botrytis.
Nitrogen is the most critical plant nutrient and controls the speed of growth of plants. Nitrogen feeding should be balanced. Too much nitrogen results in weak soft growth that is very insect and disease prone.
Sulphur is responsible for the SIR (sulphur induced resistance) reaction in plants, and the production of phytoalexins. The sulphur: nitrogen ratio is important for disease resistance. Too much nitrogen, low soil sulphur or waterlogging reduce the S:N ratio and encourage botrytis and other plant diseases.
Copper is essential for enzymes systems, photosynthesis plant respiration and metabolism of carbohydrates and proteins. It is very important for lignin synthesis and the strength and size of the xylem and phloem. Low copper results in weak floppy pale stems, premature wilting and low disease resistance.
Silicon increases stem strength and is important for healthy xylems and phloem. It thickens and hardens the waxy layer, strengthens plants, induces plant immunity, and is used as a ‘plaster’ on wounds. It is more important in monocots (grasses) than dicots. Too much silicon stunts plants and makes the waxy layer brittle. Sadly, not enough work has been done on silicon to determine optimum levels. However, you may get a good response to silicon foliar sprays if the silicon in your soil or water is low.
Sodium can be very toxic in plants, competing with calcium, potassium and magnesium, increasing the soil EC, burning root tips, disrupting metabolic processes and causing plant stress and cell burst. If you have persistent problems with plant diseases it is well worth checking sodium levels in the soil/water/media and putting a plant health program in place to deal with any problems.
The other plant nutrients also affect the health and vigour and disease resistance of plants to a lesser extent. It is important to monitor soil and plant health with regular analysis and adjust fertiliser application scientifically. For information on how to test your plants and soils please contact support@cropnuts.com.
At certain times of the year over 60% of Kenyan roses are discarded due to botrytis, causing a major problem with quality and future pricing. Botrytis is genetically plastic, and quickly builds up resistance to fungicides. We are not seeing many new actives on the market to counteract this resistance build up. It is important for farms to make informed nutritional, cultural, biological and supply chain management choices in their battle against botrytis. To this end CropNuts, Flowerwatch and Dudutech have joined forces to offer a Covid compliant, NITA Approved, one day workshop on botrytis management. For more information on this course and to book please contact billy@flowerwatch.com.
