Cole Crops, Root, Bulb and Leafy Vegetable Edition by
Jennifer Allen, Vegetable Crop Specialist, OMAFRA, Guelph

Aster Leafhoppers versus Potato Leafhoppers

It’s really important to distinguish between these two species of leafhoppers.  Aster leafhoppers have 6 black spots on their head and are often dark green in colour.  Conversely, potato leafhoppers are bright green in colour with white markings between their eyes.  In carrots, celery and lettuce, it’s important to monitor and control aster leafhoppers because they persistently transmit, aster yellows, a mycoplasma-like organism that can severely damage these crops.  Aster leafhoppers pick up the disease from feeding on infected material.   Once ingested, the disease incubates within the leafhopper for approximately 3 weeks, following which the leafhopper becomes infective and is capable of transmitting the disease for the remainder of its life.  Unfortunately once a plant is infected there are no curative measures

Since we can’t battle aster yellows directly, we target the vector, aster leafhopper.  Thresholds for aster leafhoppers are based on three factors:  1)  number of leafhoppers per 100 sweeps;  2)  crop tolerance to aster yellows (see Table 1); and, 3)  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, or as populations migrate or move with storm systems from the U.S.

For a complete listing of crop thresholds, check out Publication 363.  

Degree Days

Here are the degree and growing degree-day accumulations for April 1 – June 12, 2009. 

Area	Onion Maggot	Cabbage Maggot	Carrot Rust Fly	Carrot Weevil	Tarnished  Plant Bug	Leaf-hopper	GDD
Bradford/ Holland Marsh*	653.4	n/a	730.9	429.6	157.5	305.5	576.4
Sarnia	653.9	506.2	732.1	437.9	169.4	314.4	579.0
London	737.6	583.3	818.1	510.5	213.9	378.5	641.0
Waterloo	643.2	496.1	721.5	428.7	160.0	309.4	568.0
Windsor	931.2	769.4	1015.1	692.4	349.4	547.3	847.0

Degree-days information for pests

	Degree Days
	1st Generation	2nd Generation	3rd Generation
Onion Maggot	210	1025	1772
Cabbage Maggot	314-398	847-960	1446-1604
Carrot Rust Fly	329 – 395	1399-1711	n/a
Carrot Weevil	138 – 156 455 (90% oviposition)	No model available	n/a
Tarnished Plant Bug	40	n/a	n/a
Aster Leafhopper	128	390	n/a

Disease Forecast

BOTCAST —  Disease severity index of 12.  Risk of developing Botrytis on onions at this time is low

DOWNCAST—Predicted no sporulation infection period for the last three days.  Risk of downy mildew on onions is low.

BREMCAST—Predicted no sporulation infection period for the last three days.  Risk of downy mildew on lettuce is low.

Data provided thanks to funding from the Bradford Co-op Storage Ltd., and Muck Crops Research Station, Department of Plant Agriculture, University of Guelph

Swede Midge

Swede midge numbers have exploded on pheromone traps in Waterloo and Wellington counties.  If transplants weren’t treated with Intercept in the greenhouse, consider initiating your treatment program.