How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS AND SIGNS

Aboveground symptoms of stem canker include uneven stands, weak shoot growth, and aerial tubers. Foliage may develop yellowing, purpling, and upward curling of leaves. Aboveground symptoms alone are not diagnostic, however, because other diseases can cause similar symptoms. On belowground stems and stolons, Rhizoctonia solani typically causes reddish-brown lesions that often develop into sunken cankers. Stolons can be girdled and killed, resulting in a pruning effect and malformation and abortion of tubers. If tubers in affected fields are left in the ground after vine death, they often develop black scurf, an accumulation of irregular black sclerotia of R. solani on the tuber surface.

COMMENTS ON THE DISEASE

The strain of R. solani that typically affects potatoes, called AG-3, is specific for potatoes and generally does not cause damage in or reproduce on other species. Close relatives of potato, such as black nightshade and tomato, may be exceptions. Rhizoctonia solani is a soilborne fungus, but inoculum of the fungus on seed tubers (visible as dark, irregularly shaped sclerotia) is sometimes more important for disease development than inoculum in soil. In Kern County, AG-3 types of R. solani apparently do not survive in soil between crops of potatoes; whereas in Tulelake districts, R. solani AG-3 may overwinter in soil. The fungus only infects juvenile tissue. Disease development is favored by relatively wet, cool (55° to 60°F) soils.

MANAGEMENT

Reduce initial inoculum by using certified seed tubers that are free from sclerotia of the fungus. Where R. solani AG-3 survives in soil between potato crops, rotate out of potatoes for 2 to 3 years to reduce soilborne inoculum. Sugarbeet, however, has been associated with increased severity of stem canker in subsequent potato crops. Rhizoctonia stem canker can be reduced by practices that favor rapid emergence, such as warming seed tubers before planting, planting tubers at a relatively shallow depth, and avoiding early planting dates when soil temperatures are cool. Black scurf development on daughter tubers is minimized by harvesting quickly after vine desiccation rather than holding tubers in soil for extended periods.

Fungicide treatment of seed tubers can reduce infection by R. solani from inoculum borne on the seed pieces. The significance of this benefit may be small in fields where heavy soil infestations of R. solani AG-3 persist between potato crops.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
When choosing a pesticide, consider information relating to the impact on environmental quality Not all registered pesticides are listed. Always read label of product being used.
A.	FLUDIOXONIL
	(Maxim Potato Seed Protectant)	0.5 lb/cwt cut seed pieces	12	0
	MODE OF ACTION GROUP NAME (NUMBER1): Phenylpyrrole (12)
B.	FLUTOLANIL
	(MonCoat MZ)	0.75–1 lb/cwt cut seed pieces	12	0
	MODE OF ACTION GROUP NAME (NUMBER1): Carboxamide (7)
C.	THIOPHANATE-METHYL/MANCOZEB
	(Tops-MZ)	1 lb/100 lb cut seed pieces	12	0
	MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)/Multi-site contact (M3)
+  Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. 1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

PUBLICATION

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Diseases
R. M. Davis, Plant Pathology, UC Davis
J. Nuñez, UC Cooperative Extension, Kern Co.
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the disease section:
C. Smart, Plant Pathology, UC Davis

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