VEGETABLE CROP UPDATE

Cole Crops, Roots, Bulbs and Leafy Vegetables

Jennifer Allen, OMAFRA Vegetable Crop Specialist

SEASON SUMMARY 2006 

Vegetable Crop Updates
Table of Contents of 2006 Issues
Aster Leafhoppers in Carrots
Onion thrips
Extensive Garlic Damage in 2006
New Factsheets
Vegetable Crop Updates 

When there are crop production issues and events affecting vegetable growers and the vegetable industry in Ontario, look for the OMAFRA Vegetable Crop Updates for information.  Several editions are published:  Cole Crops, Roots, Bulbs & Leafy Vegetables; Sweet Corn, Beans & Peas; Tomato & Pepper; and others as required, such as the Vine Crop Edition.

The updates, put together by OMAFRA vegetable crop specialists, provide timely information on crop production, pest identification, pest control, pest monitoring, weather, changes to pesticide registrations, meetings, conferences, new publications of interest to the vegetable industry, and more.  They are issued whenever there is information to deliver – averaging about once a week during the growing season and less often during the winter.

2006 Vegetable Crop Updates – Cole Crops, Roots, Bulbs and Leafy Vegetables  – Table of Contents

Subscribe to all Vegetable Crop Updates online at http://apps.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp#Vegetabe or for a specific one by e-mailing jennifer.allen@ontario.ca (specify the crops you’re interested in).

Aster Leafhoppers in Carrots

As many of you know, the aster leafhopper, Macrosteles facifrons, is a pest of carrots, celery and other leafy greens.  Over the season I had a few calls asking about infectivity levels for this particular pest.  

Thresholds for aster leafhoppers are based on three factors:

• number of leafhoppers per 100 sweeps                    
• crop tolerance to aster yellows (see Table 1); and,                    
• infectivity rate

The infectivity rate for aster yellows can accurately be determined using PCR technology.  In the U.S,  leafhoppers are sampled from different locations and sent off to a diagnostic lab where the infectivity rate (the number of samples containing aster yellows) is determined, providing both temporal and spatial infectivity levels.  In Ontario, growers and consultants use either a set infectivity level of 4-5% or rely on data reported from Michigan, Wisconsin or New York.  In general, infectivity rates increase throughout the season as resident populations pickup aster yellows from infected hosts (including weed hosts), or as populations migrate or move with storm systems from the U.S.   

Aster yellows is persistently transmitted by aster leafhoppers.  Firstly, the leafhopper must feed on an infected host.  Once ingested, the mycoplasma-like organism that causes aster yellows must incubate for approximately 3 weeks before the leafhopper is capable of transmitting the disease.

Table 1.  Crop Tolerance and Aster Yellows Index (AYI) Values

Fig. 2.  Rotting of basal plate due to stem and bulb nematode infection.

Carrot Crop Tolerance                            AYI

Resistant                                                     100

• Enterprise                    
• Six Pack II                    
• Amtou                    
• Charger                    
• El Presidente                    
• Gold King                    
• Hi Color 9                    
• Impak                    
• Nanton                    
• Royal Chantenay                    
• Scarlet Nantes                    
• Toudo                       
• Enterprise                    
• Six Pack II                    
• Amtou                    
• Charger                    
• El Presidente                    
• Gold King                    
• Hi Color 9                    
• Impak                    
• Nanton                    
• Royal Chantenay                    
• Scarlet Nantes                    
• Toudo    

Intermediate                                                    70

• Carson                   
• Casey                    
• Bolero                    
• Chantenay                    
• Goldpac G                    
• Goldpac 28                    
• Long Imperator                    
• Nanco                    
• Pakmor                    
• Spartan Fancy                    
• Triple Gold

Susceptible                                                   50

• Danvers 126                    
• Indiana                    
• Heritage                    
• Spartan Bonus                    
• Bonaza                    
• Candy Pak                    
• Goldpak                    
• Lucky’s Gold                    
• Nantes                    
• Orlando Gold

References for carrot crop tolerance:  
Wyman and Granadino.  2003.  Department of Entomology, University of Wisconsin-Madison and the College of Agricultural and Life Sciences, University of Wisconsin.

Onion thrips

Onion thrips populations steadily rose throughout the 2006 growing season.  While timely rains helped to knock down some populations, thrips asexual reproduction allowed numbers to build-up quickly.  

There are a number of products registered for thrips control in onions.  Unfortunately, they fall into only three insecticide classes, pyrethroids, carbamates and organophosphates (OPs).  Therefore, rotation between classes and among products is important to reduce the occurrence of insecticide resistance throughout the season.  Field research in Ontario has shown that some of the OPs cause a ‘rebound’ effect following application.  In trials I conducted, seven days after plots were sprayed with an OP the number of thrips skyrocketed.   After doing some reading, I found that this was common in a number of crops.  OPs and carbamates are very broad spectrum, controlling targeted pests as well as other insects, including beneficials, on sprayed plants.  With no natural enemies, or competitors for food and territory remaining, thrips populations were able to build up quickly.  

For best results when applying any of the recommended control agents, spray early in the morning or later in the evening following sunset.  Dew in the morning will help get the insecticide into the leaf axils where the thrips are.  Evening spraying, when temperatures have cooled off from the daily highs helps to maximize the efficacy of the pyrethroids.   

The other key is to understand your target.  Control agents are only effective if they come in contact with the intended target.  While good coverage of the entire plant is important for fungicides, that’s not always the case for insecticides.   The majority of onion thrips on a given plant reside in the new growth or leaf axil, making it critical to target sprays into the axils.  Research in Ontario, using a fluorescent dye, has demonstrated that nozzle type, water volume and the use of surfactants can result in more or less product being delivered into the target site (Fig. 1).  Flat fan nozzles delivered more fluorescent dye to the target site than any of the other nozzles tested.  Increased water volumes with the addition of surfactants (e.g. LI-700 and Agral 90), resulted in more product being delivered to the target site.   

Fig. 1.  An onion plant sprayed with a fluorescent dye under (a) normal light;
(b) ultraviolet light.

Figure 1a

Figure 1b.

Regardless of which product or delivery method you choose, it’s important to note that adult female thrips lay eggs within the leaf tissue, protected from pesticide applications.  Therefore, it is important to monitor fields closely after each spray application as new thrips may emerge.    

If you have any questions or would like to talk about thrips control options please give me a call.

Extensive Garlic Damage in 2006

Garlic growers across the province suffered severe losses due to stem and bulb nematode during 2006.  Stem and bulb nematode (also referred to as bulb and stem nematode), Ditylenchus dipsaci, was first noted in Ontario in 1957.  Although this pest has been recorded for almost 50 years, this is the first year that there have been so many reports of losses in both small and large garlic plantings from the east, north and southwestern areas of the province.  

What is the stem and bulb nematode?

This nematode is different from other common nematodes like root-lesion or root-knot nematode because it is not confined to the root region of plants.  Stem and bulb nematodes can enter seed, invade the basal plate of the bulb, enter host leaves through stomata or move upwards into leaves through the bulb.  A unique characteristic of this nematode is that it’s capable of living without water and tolerates desiccation for several years.  

The stem and bulb nematode has three lifestages: 1) egg, 2) juvenile and 3) adult.  The juvenile lifestage is broken into four stages, 1^st stage larva, 2^nd stage larva, and 3^rd and 4^th stage larva.   Only the 4^th stage larva is capable of infecting its host.  

What does the damage look like?

Depending on when the stem and bulb nematode infects garlic, there can be a variety of symptoms.  

Early in the season, young seedlings infected with nematodes are often stunted, with chlorisis and bloating of young leaves.  Later infections can cause twisting of new growth, bulb softening and desiccation, loss of roots (Fig. 1), followed by clove destruction (Fig. 2).

Fig. 1.  Underdeveloped garlic bulb; absence of roots on one side of basal plate indicative of stem and bulb nematode infection.  

Fig. 2.  Rotting of basal plate due to stem and bulb nematode infection.

Why is this an issue for garlic growers?

Garlic is not planted from true seed. Garlic growers often keep a supply of harvested bulbs, and plant individual cloves from these bulbs in the fall. Therefore, a small infestation can quickly multiple when contaminated cloves are replanted.    Unfortunately, with a microscopic pest like the stem and bulb nematode, even plants that appear perfectly healthy when they go into storage may harbour some parasitic nematodes. If you think you have stem and bulb nematode it is important to have both your soil and garlic tested.  

For more information on hosts, biology, and recommendations for control, check out Bulb and Stem Nematode in Onions and Carrots (Factsheet 258/658; Order No. 00-043) or online at www.omafra.gov.on.ca/english/crops/facts/00-043.html.  For information on laboratory testing please contact me.  

New Factsheets

• Sampling Soil and Roots for Plant Parasitic Nematodes (Agdex 628, Order No. 06-099) or online at http://www.omafra.gov.on.ca/english/crops/facts/06-099.htm                    
• Soil Sampling and Analysis for Managing Crop Nutrients (Agdex 533, Order No. 06-031) or online at http://www.omafra.gov.on.ca/english/engineer/facts/06-031.htm                    
• Introduction to Organic Farming (Agdex 100/10, Order No. 06-103) or online at http://www.omafra.gov.on.ca/english/crops/facts/06-103.htm