When your peas have reached V10 (10th node stage), it is an ideal time to start scouting each field to evaluate if a fungicide application to manage Mycosphaerella blight is warranted. Continue scouting for symptoms from V10 (10th true node) to R2 (beginning bloom), during mid-June to late July.
Mycosphaerella (Ascochyta) blight is the most widespread and economically damaging foliar disease of Manitoba field peas. Peas are the single host crop of Mycosphaerella but it can be managed by foliar fungicide.
This pathogen can be stubble-, air-, soil- and seed-borne. Spores can travel long distances by air, meaning there is a disease risk even in fields where peas have not been grown previously.
The impact of disease severity on yield will depend on how early the disease sets in and how quickly it progresses into the upper crop canopy. Early infections during the bloom to early/mid-pod stages cause the most damage if left untreated.
Use this fungicide decision worksheet when scouting to determine if fungicide application is necessary.
It is a great idea to down load the
Pro tip: If you will be scouting several pea fields, from the download, screen shot a picture of the handy rating scale to your mobile device several times.
First, evaluate your crop canopy. Throw your trusty 28.25 inch diameter hula hoop in the crop. Push it down into the crop where it lands and pull out each pea plant inside the hoop to count. You will also be able to get a good look at the pea leaves on each plant. If you have 34 plants in your hoop, you have approximately 8 pea plants/ square foot, which is considered a normal crop canopy. Record the number of plants per hoop, as you will need that number for a step on the decision worksheet. Estimate the average amount of weeds present in the field. Consider if the ground is well covered with pea plants or if it is a fairly thin canopy.
Next, have a look at your pants to see how wet they are as well as your boots/shoes to see if they are picking up mud from walking through the field. If you are at the field earlier or later than noon, take your best guess as to what conditions would be.
Then tap your favorite weather app and see if there is rain coming in the next five days.
Pick up each pea plant that you pulled up from inside your hula hoop and closely examine the lower leaves for signs of “purple/ dark brown freckles”. Sometimes it helps to look at the plant upright and then turn it upside down and give it a twirl to get a good look at all the leaf surfaces. You may want to pluck off individual leaves to examine them closely.
Count the total number of plants that have freckles or brown lesions with concentric rings. Divide this by the total number of plants that were in your hula hoop to determine the % of plants showing symptoms. Below are the symptoms that you are looking for:
Examine leaves closely because Bacterial blight can look similar to Mycosphaerella blight, but they are different.
Pro Tip: In your camera roll, on one of the pictures you saved of the handy rating scale, use the editing function and choose the marker. Circle on the picture your values for that field, record and total the points from each section, and write the field name at the top and resave back to your camera roll.
If your total score is 65 or higher, then it is recommended to proceed with a fungicide application.
Cool, wet conditions promote disease development. If favourable conditions persist, lesions will expand, produce spores and disease will develop further up the plants. The risk of yield loss increases when symptoms progress above the bottom third of the plant canopy by the R2 stage. Monitor closely for any upward movement of symptoms within the crop.
Foliar fungicide application for control of Mycosphaerella should be made at R2, when one flower is open on most plants across the field. The goal is to achieve adequate fungicide coverage on leaves in the lower canopy. This means spraying just before the canopy begins to close. If disease symptoms are not yet present, it may be beneficial to delay fungicide application (for example, in a dry year).
If symptoms spread into the mid to upper canopy and warm, humid weather persists, consider a second fungicide application 10 to 14 days later using a different mode of action.
Once peas reach R4 (full pod) fungicide application is no longer recommended. At this stage, peas are within the 30-day pre-harvest interval of several products and disease is no longer expected to be yield-limiting.
Managing Ascochyta (Mycosphaerella) Blight in Field Peas – Manitoba Pulse & Soybean Growers