Phytophthora
Registered against on 7 crops
Phytophthora species—including Phytophthora cinnamomi, Phytophthora nicotianae, and Phytophthora citrophthora—are soil-borne water moulds (oomycetes) that cause destructive root rot, crown rot, collar rot, and fruit rot across key Australian tree crops and horticultural plantings.
The economic impact on growers stems from reduced root surface area, which severely restricts water and nutrient uptake. Infected orchards show gradual foliage decline, reduced fruit size, and canopy dieback, while severe stem or crown rot can kill established trees and tomato plants rapidly. Over time, unmanaged infections drastically shorten orchard lifespan and require costly block renovation.
Where it matters in Australia
Phytophthora is a major pathogen across diverse production districts in Australia. In warm, high-rainfall regions like the Northern Rivers, Bundaberg, and the Atherton Tableland, Phytophthora root rot and trunk canker pose constant pressure on macadamia, avocado, and citrus orchards during wet summer months. In irrigated southern regions such as Sunraysia, the Riverland, and the Goulburn Valley, collar rot and root rot impact citrus and pome fruit (apples) following heavy rain, soil compaction, or temporary waterlogging in low-lying orchard blocks. Processing and fresh-market tomato paddocks across coastal and inland growing districts are vulnerable during warm, humid periods.
Symptoms
Symptoms vary by host and plant tissue, but key features distinguish Phytophthora from general root stress or nutrition issues:
- Citrus (lemons, limes, oranges): Root cortex sloughs off easily, leaving exposed vascular threads. Trunk collar rot causes cracked bark with dark amber gum oozing from trunk bases (gummosis).
- Avocado and Macadamia: Feeder roots turn black and brittle. Foliage thins, leaves turn pale green to yellow, and branch tips die back ("stag-heading"). Trunk cankers produce pale or dark exudate.
- Apple: Crown rot shows reddish-brown, moist wood decay beneath the bark near or below the soil line.
- Tomato: Seedlings damp off, while mature plants show sudden Wilting, dark brown stem lesions near the soil line, and firm, dark water-soaked rot on fruit.
Life cycle
Phytophthora overwinters and survives dry soil conditions as thick-walled resting structures (chlamydospores or oospores) in diseased root tissue and soil. When soil temperatures warm and free water is present, these resting spores germinate to produce sporangia, which release mobile, swimming zoospores into water-saturated soil. Zoospores are attracted to root tips and stem bases, where they encyst, germinate, and invade plant tissue, rotting the outer root cortex.
Secondary spread occurs rapidly through irrigation water, soil movement on machinery, surface run-off during heavy rain, and infected nursery stock. Splash from rain or overhead irrigation can also carry spores onto low-hanging fruit and lower trunks, initiating aerial infections.
Monitoring
Scout blocks regularly, prioritizing low-lying, poorly drained, or heavy clay areas, as well as row ends near irrigation valves. Inspect tree canopies for unseasonal yellowing, thin foliage, or tip dieback. Excavate feeder roots near the dripline; healthy roots are firm and creamy white, whereas infected roots are dark brown or black, water-soaked, and slough away when rolled between fingers.
In citrus and pome fruit, check trunk bases for dark, water-soaked lesions or gum exudation. In tomato paddocks, check seedling stems near the soil surface for water-soaked lesions after rain or irrigation. On-site diagnostic test strips or accredited laboratory testing of soil and root samples can confirm Phytophthora before extensive canopy symptoms appear.
Management
Managing Phytophthora relies on an integrated strategy focusing on drainage, orchard sanitation, and targeted chemical applications:
- Cultural control: Improve soil structure and drainage by planting on raised mounds or beds and installing subsurface drainage where needed. Use soil moisture monitoring to avoid over-irrigation. Plant certified disease-free nursery trees onto tolerant rootstocks. Practice strict farm biosecurity by cleaning soil from boots, tractors, and implements before moving between paddocks.
- Biological control: Maintain high soil organic matter using mulches and composts to promote antagonistic soil microorganisms that suppress spore survival and encourage root regeneration.
- Chemical control: Use registered systemic and protectant fungicides integrated into your spray diary, particularly during key root flushing periods. Do not rely on chemical control alone; chemical treatments are most effective when paired with good drainage. Always consult the approved label for application methods, crop safety warnings, and withholding periods (WHPs). Rotate active constituent groups to prevent pathogen resistance.
Crops with registered control options
Common questions
- How do I distinguish Phytophthora root rot from waterlogging damage?
- Waterlogging alone causes roots to soften without the characteristic sloughing where the outer root cortex easily peels away from the central vascular core. Phytophthora is also typically accompanied by distinct stem or trunk lesions, bark cracking, or amber gum exudation above the soil line. Laboratory testing or rapid test kits provide positive confirmation.
- Where do I find specific application rates and withholding periods for my crop?
- Application rates, water volumes, maximum seasonal applications, and withholding periods (WHP) are strictly detailed on the APVMA-approved label on the product container. Always read and follow the label directions specific to your crop and state before application.
- When is the best time in the season to apply systemic fungicides for Phytophthora?
- Systemic fungicides are generally most effective when trees are actively translocating sap during spring and autumn root flushes. Applying when roots are growing allows systemic actives to be absorbed and transported throughout the root system. Check the product label for specific timing and crop stage recommendations.
- Why does rootstock choice matter so much for tree crops?
- Tolerant rootstocks reduce the severity of root cortex infection and allow trees to maintain root function under moderate disease pressure. Combining tolerant rootstocks with raised planting mounds provides the most effective long-term defense against Phytophthora root rot.
- How should I manage chemical resistance for Phytophthora products?
- Avoid relying exclusively on a single fungicide mode-of-action group across consecutive seasons. Alternate between registered chemical groups in your spray diary, apply treatments protectively before root systems suffer severe decline, and integrate cultural controls like drainage and mounding.
- Why are there different numbers of registered products across crops listed on the site?
- The variation in registered products reflects commercial crop size, historical registration filings by chemical manufacturers, and specific regional disease pressure. High-acreage perennial tree crops like citrus and avocado have broad product availability, whereas other crops may have fewer options. Always confirm the product label lists your specific crop before use.