- Author: Jennifer Magana
Identification of Pythium wilt of lettuce in the field
Authors: Richard Smith, JP Dundore Arias and Steve Koike
Farm Advisor, UCCE Monterey County, Professor of Plant Pathology, California State University, Monterey Bay, and Director, TriCal Diagnostics
Pythium wilt, caused by Pythium uncinulatum, infects the roots of different types of lettuce plants. The pathogen survives in the soil in the form of resting spores called oospores, which are resistant to desiccation and temperature changes, and can survive in the soil for several years even in the absence of the host. Upon detecting the presence of the crop, oospores can germinate and infect plant roots directly, or form swimming zoospores, which can spread through water towards roots of the host plant. They attach to and infect the roots. Given that the pathogen needs saturated soil conditions to initiate infection, disease is often most prevalent in wetter parts of the field such as at the head or tail ends of the rows. Under severe infestations and especially with susceptible lettuce varieties, it can occur throughout the planting.
Pythium wilt has some distinctive characteristics that help in recognizing it in the field. However, there are other diseases that overlap with some of the Pythium wilt symptoms. Below is a guide for recognizing Pythium wilt in the field and to help distinguish it from other soilborne diseases. Be sure to wash the roots off to get the best view of the symptoms.
- Pythium wilt (Pythium uncinulatum):
o Leaves: Older leaves wilt, turn yellow, collapse and turn brown; no distinct lesions, spots or specking (as seen with INSV); younger leaves remain upright, but in advanced stages all foliage collapses.
o Crowns: Do not have soft, watery decay. Mostly remain intact. Plants are not easily pulled from the ground (crowns to not readily break off).
o Roots: Taproot exterior can be distinctly dark brown to black, tissue rotted; feeder roots partially or extensively dark brown to black (discoloration does not occur in discrete bands or sections).
Older leaves of Pythium wilt infected plants collapse onto the ground; younger leaves often remain upright until complete death of the plant. Plants do not easily pull from ground.
Pythium wilt: Outer tissue of the taproot (left) or feeder roots (middle and right) turn dark and are rotted.
- Lettuce Drop (Sclerotinia minor):
o Leaves: All leaves wilt, turn yellow, collapse and turn brown; no distinct lesions, spots or specking (as seen with INSV).
o Crowns: Have a soft, watery decay and become mushy in advanced stages; white to gray mycelium and black sclerotia (usually 1/8 inch in diameter) form on the crowns. Crowns readily break off from taproots.
o Roots: Rarely infected and therefore show no disease symptoms.
Lettuce drop infected plants have soft watery, decayed crown tissue and readily break off from the taproots when the plant is gently tugged.
- Botrytis Crown Rot (Botrytis cinerea):
o Leaves: All leaves wilt, turn yellow, collapse and turn brown; no distinct lesions, spots or specking (as seen with INSV).
o Crowns: Have a soft, watery decay and become mushy in advanced stages; crown decay is often an orange-brown color; white to gray mycelium initially forms on the crowns; later becomes gray and fuzzy. Large black sclerotia may also form on decayed crowns. Crowns readily break off from taproots.
o Roots: Are not infected and show no disease symptoms.
Botrytis crown rot attacks the crown tissue. Masses of gray mycelia are evident on infected tissue.
- Verticillium wilt (Verticillium dahliae):
o Leaves: Older leaves wilt and turn yellow. Leaf collapse occurs as plants mature and reach harvest stage.
o Crowns: Show black discoloration in the vascular tissue but not in the central core/pith.
o Roots: Taproot exterior is not discolored. Feeder roots are not discolored. Cut taproot lengthwise to show black streaking of the root vascular tissue. However, the black streaking is firm and not decayed. The vascular discoloration can be similar with symptoms caused by Fusarium wilt and ammonium toxicity.
o Other: Because plants develop symptoms near harvest, plants are rarely stunted. Head lettuce usually is the most susceptible lettuce type.
Verticillium becomes evident close to harvest. Black streaking is evident inside the tap root.
- Fusarium wilt (Fusarium oxysporum f. sp. lactucae):
o Leaves: Older leaves wilt and turn yellow; symptoms can occur on both younger and older plants.
o Crowns: Show brown to red discoloration in the vascular tissue but not in the central core/pith.
o Roots: Taproot exterior is not discolored. Feeder roots are not discolored. Cut taproot lengthwise to show brown to red streaking of the root vascular tissue. However, the brown/red streaking is firm and not decayed. The vascular discoloration can be similar with symptoms caused by Verticillium wilt and ammonium toxicity.
o Other: If infected early in development, plants may be severely stunted. If infected later in the crop cycle, plants may reach full, normal size.
Fusarium infected plants often occur in patches and are stunted with outer leaves turning yellow and necrotic. Tissue turns reddish-brown and breaks down inside the root.
- Ammonium toxicity is not a disease but is a physiological disorder. Lettuce roots are reacting to elevated levels of ammonium that build up in the soil due to cool temperatures in the spring:
o Leaves: Older leaves wilt and turn yellow; symptoms most often occur on younger plants (before and up to rosette stage).
o Crowns: Usually show no symptoms.
o Roots: Taproot exterior is not discolored. Feeder roots are not discolored. Cut taproot lengthwise to show brown to red streaking of the root vascular tissue and central root core. However, the brown/red streaking is firm and not decayed. Central pith of the root can break down and become hollow. The vascular discoloration can be similar with symptoms caused by Fusarium wilt.
o Other: Plants with ammonium toxicity damage usually are distributed randomly and singly throughout a field, in contrast with soilborne disease problems which occur in groups and patches in the field.
Ammonium toxicity mostly occurs in the spring. Plants are often scattered in a field and often, but not always, wilt in the afternoon.
These descriptions can help you to get a good idea of which disease you are dealing with. In spite of the some of the good characteristics of Pythium wilt, getting a sample to a diagnostic laboratory is the best way to be sure of which disease you are dealing with.
- Author: Larry J Bettiga
Airblast 2021: Optimizing Canopy Sprayers webinar
Speaker bios, agenda and registration at: https://ucanr.edu/sites/Airblast2021//
Monday, March 29, and Tuesday, March 30 11:00 AM-2:30 PM Pacific Time
$50 USD registration fee increases to $75 USD on March 22, so register early!
Co-organized by the US/Canadian Spray Application Working Groupand University of California Cooperative Extension
A two-day webinar providing participants with practical information to maximize the effectiveness and efficiency of their air-assisted sprayers used in orchards, vineyards and some bush crops.
Intended Audience:Pesticide applicators, supervisors, growers, licensed professional applicators, Pest Control Advisors and anyone involved in spray application decision making in the orchard or vineyard.
Webinar Objective:Featuring recognized experts hailing from the U.S. and Canada, this unique nationally broadcast workshop, the first of its kind, aims to provide participants the knowledge and insight needed to assess and improve their own spray operations. Presentations will use a variety of air-assist sprayers used in a wide range of orchard/vineyard cropping systems. Audience interaction via Q&A sessions at the end of each day is welcome.
Pesticide Continuing Education Units (CEUs) are applied for the following: California, Oregon, Michigan, New Hampshire and Washington and British Columbia, Canada.
Virtual Vertebrate Pest Council Webinars
This meeting will be three 2-hour sessions spread across three days. Each day will have a different theme:
Day 1 (March 30): Field rodents.
Day 2 (March 31): Updates and information on the use of rodenticides.
Day 3 (April 1): Managing commensal rodents.
These are unique seminars in that there generally is not any other place where you can get continuing education credits exclusively for vertebrate species. Experts from around the state are lined up to provide these seminars. The link for the website with all the details can be found here: http://www.vpconference.org/. A flyer advertising the seminars/workshops can be found here as well: https://ucanr.edu/sites/VPC/files/345087.pdf.
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- Author: Michael D Cahn
The rain situation was beginning to look dire for our region before last week, with most major storms passing to the north of Monterey county. However, the storms that occurred last week were generated by an atmospheric river that was focused on the southern part of Monterey County. Cumulative depths recorded at CIMIS weather stations along the valley showed increasing amounts moving south in the valley with almost 9 inches recorded at the King City CIMIS weather station (station 113) (Fig. 1). Also 8.3 inches were recorded at San Antonio and Nacimiento Reservoirs, where before these storms less than an inch of rain had fallen since October.
This one weather event was able to significantly increase the water stored in Nacimiento reservoir and helped the situation in San Antonio (Fig. 2.) Nacimiento water storage increased from 21% to 41% between January 23 and February 3, and San Antonio increased from 16% to 20% capacity. In combination, water stored in the two reservoirs increased from 133,778 acre-ft to 216,858 acre-ft, representing 64% more water compared to before the storm events. Total capacity of the two reservoirs is 712,900 acre-ft, so water stored in the reservoirs at this point in the season is still at 30% of maximum capacity.
Our region usually receives a few atmospheric river events each winter, most of which usually pass too far north or south to greatly impact the Salinas Valley reservoirs. This first major rain event of the season was a direct hit for the reservoirs. Hopefully, more rain will be coming in the upcoming weeks.
The other benefit of this last storm was that by mostly passing over the southern part of the county, debris flows were minimized in the burned areas.
If you want to keep track of the reservoir storage as we proceed through the winter visit the link at Monterey County Water Resource Agency website.