Phytophthora life cycle

Phytophthora cycle

Soil preparation and root diseases

Curtis Swift, Ph.D.,

Phytophthora is a major disease of lavender and becoming more of a problem due to the lack of knowledge and field preparation by growers. Proper soil preparation is critical to preventing Phytophthora root rot and other disease problems. Proper soil preparation also enhances plant growth, and yield.

Once the field is planted and infection by Phytophthora has occurred, there is little the grower can do to stop the spread of the disease. No chemicals can cure infected plants and no biological control agents provide adequate control, only suppression.  The improper preparation, design and layout of the field can result in the infection spreading and infecting many more plants.

It makes more sense to correct the issues which lead to infection before planting than trying to correct the problem once the pathogen is established in your field. You may need to allow the field to go fallow (without any crop) for 3 to 4 years before you replant. Clay and sandy soils have been found to be infectious for 34 and 48 months once contaminated with the pathogen.  Phytophthora can infect and develop on many different crops so planting an infested field with another crop may only increase the problem as the pathogen increases. The following is an outline of how to prevent root rot diseases becoming an issue in the first place.

Note: your field may already be infested with this disease. If that is the case, these suggestions will at least help prevent its spread.

Note: Planting lavender on raised beds reduces the potential for Phytophthora root rot infections as it typically improves drainage, soil aeration, and enhances root development.  Next time you see an image of plants suffering from Phytophthora pay particular attention to whether the plants are growing on raised beds or on the level.  You will seldom see plants diseased by this organism on raised beds.  Even when you use raised beds, it is critical you follow the steps outlined below to prevent an infection or spread of the disease.

Plant Phytophthora-free plants

  • Purchase from a reliable source. While this will not guarantee your plants are not already infected, it does help.
  • Check the roots and lower stems of the plant for the tell-tale symptoms of this disease. There are a number of web sites with photos which will help you to identify these symptoms. Test suspect plants for the disease.
    • Before your plants arrive, purchase diagnostic strips to test your crop. These are available from, or
    • have your plants diagnosed for phytophthora at your state’s University diagnostic lab. Dr Steve Jeffers, Clemson University is renowned for assisting lavender growers with identifying phytophthora infected plants. You can reach him at
  • If you suspect a plant is infected and don’t have the means to have it tested, don’t plant it.

Soil Preparation and Root Diseases: Correct soil drainage issues

  • Many growers mistakenly consider drainage to be only a measure of the movement of water off a site. While this is important, as you don’t want water to puddle on the soil surface and prevent oxygen from entering and preventing carbon dioxide from escaping the root zone, lavender growers also must be concerned with how quickly water moves into and down through the soil profile. As water enters and moves down through the soil, it pulls oxygen along with it. It also allows carbon dioxide to escape. This requires spaces between the soil particles. The air channels in the soil must be open all the way to the soil surface. Air spaces which are too small will prevent air movement into and down through the soil.
  • The drainage of your soil can be evaluated as follows:
    • Dig a hole 1 foot deep and 1 foot across
    • Fill the hole with water to the top
    • Allow the water to drop 6 inches
    • Fill the hole again
    • The water will drop an inch in an hour if it is well-drained
  • Soils that don’t drain adequately or which drain too quickly should be amended before planting.
  • Amending with the proper material and size of material is critical. Selecting the wrong material or size of material can make things worse in the long haul.

Air infiltration – oxygen in and carbon dioxide out

  • Oxygen is required for root growth. Oxygen is used to ‘burn’ the sugars and starches in the cells to keep those cells functioning just as your cells require oxygen.
  • As oxygen is consumed by roots, carbon dioxide is released and can build up in the soil. This carbon dioxide must escape from the soil. As soil carbon dioxide levels increase, roots become more susceptible to disease pathogens such as the fungal-like organism Phytophthora. This organism is known to move (swim) to areas with low oxygen/high carbon dioxide levels, i.e. to roots.
  • In addition to poor soil drainage, other issues can trap carbon dioxide in the soil and prevent oxygen from entering.
    1. Pulverizing the soil prior to planting will destroy the open channels. When tilling soil, avoid the tendency to continue tilling once the soil clods are smaller than one inch in diameter.
    2. Applying a tightly woven landscape fabric over the planting row. This is especially true when the area is subject to winds that carry dust onto the fabric plugging the aeration holes.
    3. Neglecting to apply a coarse mulch over the soil. This is important when a landscape fabric is not used. Rain droplets and droplets from overhead irrigation can result in a layer of clay forming on the soil surface blocking air channels.
    4. Irrigating the field when the water supply contains large quantities of fine soil and/or organic particles.
  • Amend the soil to improve drainage.

Soil Preparation and Root Diseases: Proper soil fertility is critical 

  • Plants require a balance of nutrients to be healthy and disease-free. A small imbalance in the soil nutrient level can have adverse repercussions on lavender. Thus, determining the fertilizer needs and applications, whether from organic or inorganic sources is critical.
  • pH – this measurement refers to the ‘Power of Hydrogen’ in the soil. Lavender would like a pH between 6.7 and 7.3. To correct a soil with the wrong pH level, many neglect the necessity of applying the proper liming or acidifying product to adjust the pH. The pH level also determines the health of root protecting mycorrhizal fungi. The soil test lab should be able to specify what product and the fineness of product you should use. Since many labs consider a pH of 6 as appropriate (they are used to testing soils for field crops) you need to inform the lab you want to adjust your soil pH level to as close to 7 as possible. If they argue with you, use a different lab.
  • Phosphorus – Excessive levels of soil phosphorus and applying too much phosphorus during an application is known to negatively affect the mycorrhizal-fungi which protect the roots from attack by pathogens.
    • While mycorrhizae are typically beneficial, excess phosphorus can change this fungus from a beneficial organism to a parasite further weakening roots and enhancing a root pathogen’s potential to infect. Lavender is a mycorrhiza-dependent plant and requires this symbiotic relationship to be healthy.
    • Too little or too much available soil phosphorus directly affects the availability of other nutrients impacting plant health.
    • The extraction method used to determine available phosphorus by the lab is critical. The method the lab uses must be taken into consideration when recommending additions of phosphorus.
    • Phosphorus must be tilled into the root zone to be effective.
    • Excess levels of phosphorus must be corrected by improving drainage.
  • Soluble salts – Many soils contain saline and sodic soluble salts. These dehydrate the roots and increase the potential for infection by pathogenic microbes. This is typically reported on a soils test as Electrical conductivity and Sodium Adsorption Ratio (SAR). While lavender can tolerate some soluble salts, they have their limit.
    • For soil too high in soluble salt level, improve drainage, eliminate any hard pans within three feet of the soil surface, and irrigate with a low-salt water to flush salts well below the root zone of the plants.
    • In sodic soil (high SAR), the addition of gypsum may be necessary. Gypsum, however, should never be applied unless the SAR is excessive.
    • If using well water or affluent, have the water tested to ensure salts are not a problem.
  • Nitrogen – Many growers assume lavender does not need or want nitrogen. Nitrogen is a major component of chlorophyll, the molecule which harvests light energy to produce the foods necessary to keep the plant healthy. Without the proper amount of nitrogen, the plant’s health is negatively impacted resulting in an increase in pathogen infections. The sugars and starches produced during photosynthesis also feed the root-protecting mycorrhiza and other microbes. Without adequate nitrogen, these microbes are unable to protect the roots resulting in the potential for root disease organisms to invade. Due to the critical nature of soil N, routine soil analyses are important for the health of the plant.
    • The proper amount of soil nitrogen ensures the optimum uptake of potassium as well as phosphorus, magnesium, iron, manganese, and zinc from the soils.
    • Many university soils labs do not test for N. Selecting the proper lab to test your soil is important.
  • Potassium is responsible for the activation of over 60 enzymes in the plant system such as protein synthesis, sugar transport, N and C metabolism, photosynthesis, and others. It assists and facilitates many required plant processes and directly affects root growth and the plant’s pathogen resistance. Excess levels of soil potassium impact the availability and uptake of other plant required nutrients.
  • Micronutrients – While these make up a very small part of the plant, they are critical to plant growth and development. Imbalances can create major problems affecting plant health. Some micronutrients act as enzymes activating photosynthesis, respiration, nitrogen fixation, and many other essential plant processes. Magnesium is the center of the chlorophyll molecule. Over-applying fertilizers or organic materials can negatively impact micronutrient uptake and availability.
    • Never apply a nutrient unless a soil test shows it is deficient. This even applies to calcium products used to adjust pH.

Planting on raised beds

  • Raised beds help increase drainage, air infiltration, and the plant’s rooting area.
  • Raised beds cannot correct the soil nutrient problems noted above. These need to be corrected prior to bedding the field.
  • The slope and drainage of the lavender field must be considered when establishing the raised beds to enhance movement of excess water off the field.

Hire a qualified consultant

I’m available if you are interested. You can reach me at



No responses yet

Leave a Reply

Your email address will not be published.