Phytophthora

Root and stem rots

Pathogen

Phytophthora spp. (P. cactorum, P. drechsleri, P. nicotianae, etc.)

Hosts include

Achillea, Clematis, Euphorbia, Fragaria, Heuchera, Leucanthemum, Lilium, Phlox, Platycodon, Sedum, Sempervivum, Scabiosa and Viola.

Phytophthora stem rot lily
Aerial Phytophthora infection caused water-soaked lesions and stem collapse of lily

Symptoms

Stem and crown rot, root rot, stunting, wilting, yellowing. Infection of aboveground plant parts causes foliar dieback. Plants with low levels of infection may not have obvious symptoms.

Spread

Phytophthora is a soil-borne pathogen; spores can also be disseminated short distances through the air. Moving infested plant material can spread the disease. Phytophthora has several different spore types, including chlamydospores, sporangia, zoospores and oospores. (Refer to Pythium root rot for a description of these spore types.)

Management

Management strategies for Pythium spp. also apply to Phytophthora. Symptomatic plants should be removed and destroyed. Fungicides are often used to prevent losses. It is important to get good coverage of the affected plant parts. Fungicides have limited scope and should not be expected to cure heavily infected plants. Systemic fungicides should be used in rotation with protectant fungicides to delay resistance development. Applications at the high end of the labeled rate are required for Phytophthora control.

Phytophthora
Discolored roots caused by Phytoph-thora infection.

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