Pest of the month: Phytophthora root rot

Category: Phytophthora

Importance: Common and serious nursery pathogens, some species are exotic.

Symptoms: Phytophthora pathogens attack all parts of a plant including roots and crowns, fruit, leaves, stems, flowers and buds causing root rot, collar rot, tree canker, stem lesions, bud rot, heart rot, leaf blight, fruit rot, tuber and corm rot.

Common symptoms include rotting of roots, rotting of trunk at ground level, leaf yellowing and death, dieback, leaf drop, stunting of older plants, discolouration and wilting of the upper parts of the plant. May also cause seedling damping-off. Some species cause stem and foliage lesions.

Transmission: They develop sporangia that can be dispersed by wind and rain, and zoospores that are free swimming in soil or surface water which cause rapid pathogen spread. Hyphae, chlamydospores and oospores can be carried in infested plants, plant debris, seed and cuttings or on tools, shoes, vehicles and clothing. They are also spread by animals via ingestion of contaminated seed or externally after contact with infested material; fungus gnats can also spread zoospores. Some species may be spread in dry wind and dust.

Favoured by: Phytophthora pathogens thrive in humid and wet conditions. Poorly draining soil (saturated soils), close plant spacing that reduces airflow through the crop, constant standing surface water and stressed plants all provide an ideal environment for the pathogen to multiply. Soil temperatures about 15oC trigger germination of dormant zoospores that can swim through soil and surface water to infest other production areas.

Host range: Phytophthora spp. have a wide range of hosts including avocados, citrus and stone fruit, pineapple, papaya, taro, passionfruit, macadamia, olive, strawberry, fig, mango, asparagus, pepper, eggplant, tomato, cucumber, melon, holly, azalea, bougainvillea, dogwood, hibiscus and many more plant varieties. It can infest and reproduce in both live and dead tissue of host plants.

Further industry pest information is available at www.pestid.com.au.

An initiative of the National Nursery Industry Biosecurity Program (NY15004) and the ‘Building the resilience and on-farm biosecurity capacity of the Australian production nursery industry’ Project (NY15002).