How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS AND SIGNS

Root rots can affect all of the cucurbits at any growth stage. However, Rhizctonia and Thielaviopsis are generally limited to infecting cucurbit seedlings. In plants infected with Pythium, Phytophthora, or Rhizoctonia, the hypocotyl collapses and turns tan to brown, roots die, and cotyledons and/or leaves wilt. Infection by Acremonium or Fusarium occurs where the seed coat attaches to the hypocotyl. The hypocotyl and roots turn red-brown and the portion above swells slightly. Plants may not die, but become stunted with cotyledons and leaves turning dark green. Thielaviopsis basicola turns roots and lower hypocotyl black; cotyledons and leaves become dark green. Infections can lead to stunting, vine collapse, and death.

COMMENTS ON THE DISEASE

Pythium, Phytophthora, and Rhizoctonia require free moisture and cool soil temperatures for disease development. Planting early before soil temperatures are adequately warm or planting into poorly drained beds will likely increase disease. Melons following alfalfa generally have greater risk of infection by Pythium spp.

Acremonium spp. occurs in old melon fields in Stanislaus, Merced, Fresno, and San Joaquin counties. Disease incidence is severe when melons of any cultivar are planted without rotation. Rotation with wheat reduces incidence and severity. Planting depth also influences disease incidence. Deeply planted melons are more severely affected.

Fusarium equiseti root rot occurs in the central and south San Joaquin Valley and is more severe when melons are planted following cotton.

Fusarium solani is rarely seen but occurs occasionally on squash, pumpkin, and melons. The pathogen is most prevalent in the Colusa, Sutter, and Yuba county areas, but also has been observed on honeydew melons in Stanislaus County.

Thielaviopsis basicola does not usually infect cucurbits. Because the pathogen also attacks cotton, it is more likely to occur in the southern San Joaquin Valley. Chilling stress is necessary for disease to occur.

MANAGEMENT

Prevent root rot damage to seedlings by planting appropriately. Fungicides also provide control.

• Planting beds should be high and well drained.
• Waiting to plant until soil temperatures are 65° to 70°F will generally prevent seedling infections by Pythium, Phytophthora, or Rhizoctonia.
• For Acremonium seedling root rot, shallow planting (0.5 inch) and irrigating up is the preferred method of managing this disease.
• For Thielaviopsis root rot, plant when soil is warm.
• Use treated seed in no-till, conservation-till, and when planting through cover crops to reduce the potential for seedling diseases.
• If Pythium, Phytophthora, or Acremonium spp., or Fusarium equiseti are present, an application of fungicides,depending on the species present, provides good control.
• Plants infected with Fusarium may recover if the weather is warm and the field is irrigated enough to wet the soil surface; this allows the roots to regenerate and the plants to produce a normal, but delayed, crop.

If root rots are observed during the vegetative growth stage, note infections to make management decisions for the next crop.

Common name		Amount/Acre	R.E.I.+	P.H.I+
(trade name)			(hours)	(days)
Not all registered pesticides are listed. Always read the label of the product being used.
A.	MEFENOXAM
	(Ridomil Gold SL)	Label rates	48	5
	MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
	COMMENTS: Can be used as a soil drench or at planting treatment for Pythium and Phytophthora diseases. Use allowed under a Special Local Needs label.
B.	THIOPHANATE METHYL
	(Topsin-M)	0.5 lb	24	0
	MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
	COMMENTS: Effectively controls seedling root rots caused by Acremonium spp.; most useful in areas (Stanislaus, Merced, Fresno, and San Joaquin counties) where Acremonium is prevalent.
C.	AZOXYSTROBIN
	(Quadris)	11.0–15.5 oz	4	1
	MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENT: For Rhizoctonia solani.
D.	METALAXYL	Label rate	24	NA
	(Acquire)
	MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
	COMMENTS: Aquire is not labeled for use in California, but seed treated in and obtained from another state can be legally used in California even for a chemical not registered on cucurbits in California.
+	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. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 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.
NA	Not applicable.

PUBLICATION

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445
Diseases
R. M. Davis, Plant Pathology, UC Davis
T. A. Turini, UC Cooperative Extension, Imperial County
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to Diseases:
W. D. Gubler, Plant Pathology, UC Davis

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