Wet conditions encourage soil-borne diseases of potatoes

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

The recent and continuing stretch of cold, wet weather has resulted in soils that are water saturated and may remain cool and wet for several weeks. These conditions are particularly suitable for the development of lower stem and root-rotting pathogens for example, Phytophthora spp., Rhizoctonia, Pythium, Fusarium and other soil-inhabiting fungi and bacteria such as soft-rots (Erwinia caratovora). Spores of these fungi and water-molds can remain dormant in the soil for many years, and it is likely that with current soil conditions and retarded development and growth of roots, that many plants are vulnerable. (see photos) Infected plants may exhibit symptoms that start with wilt and apparent nutrient deficiencies and ultimately become necrotic and die. The roots lose integrity and adhesion to the soil.

To check for lower stem rot or root rot, gently but firmly pull the plant upwards, if the plant comes away from the soil with minimal resistance and the roots are darkened, root rot may be the problem. Samples should be sent immediately to the diagnostic clinic for confirmation and diagnosis of the problem in order to implement appropriate control recommendations. Some accompanying images of root and lower stem rot symptoms in potatoes are included.

It is also possible that fungicidal seed treatments and in-furrow applied fungicides are no longer present in sufficient quantity in the soil to be effective. Once the soil has dried out sufficiently (about 15 to 20% volumetric soil moisture) appropriate fungicides for soil-borne and root diseases may be applied. Use only fungicides recommended for the diagnosed pathogen and use label recommended rates and application methods (Table 1). Application of azoxystrobin (Quadris 2.08SC) shortly after emergence inhibited Rhizoctonia black scurf in trials at MSU in 1998 and has continued to be effective in more recent trials (see Figure 1).

Table 1. Fungicides for lower stem and root rot diseases. Class, common name, trade names and pathogen targets for potatoes.

Class of fungicide

Common name

Trade names

Pathogen targets

Carbamate

Propamocarb HCl

Previcur Plus

Phytophthora spp.

Coppers, unfixed

Copper Hydroxide

Kocide Champ

Topical bacteria Phytophthora spp.

Organic phosphate

Phosphonic acid

Phostrol

Pythium Phytophthora spp.

Phenylamide

Mefenoxam

Ridomil Gold and other formulations

Pythium Phytophthora spp.

Strobilurines and other group 11.

Azoxystrobin Pyraclostrobin Trifloxystrobin

Amistar Headline Gem

Rhizoctonia

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