Onion Pest Management


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Purple Blotch (Alternaria porri) Onion Disease Model

VegNet offers an interactive interface to calculate PRI values for Purple Blotch of Onion. This computer generated disease model is being investigated by Colorado State University to identify environmental conditions which favor the development of Purple Blotch in our region.

Calculate PRI value from a CoAgMet weather station near your area:

Enter the year, month, and day of emergence or transplanting of your onion crop
choose a nearby weather station
Submit your request

PRI (Daily Predictive Index) values are calculated from three factors over a three day period; Temperature Index (TI), Night time Relative Humidity Index (NRHI), and Low Relative Humidity Index (LRHI), and predict spore formation and release events of A. porri. Our preliminary research indicates that fungicide applications may not be necessary for Purple Blotch control until after 300 PRI Values have accumulated from emergence and presence of the pathogen/disease is confirmed.

Calculation of PRI:

Dr. Melvyn Lacy, Retired Professor, Department of Botany and Plant Pathology, Michigan State University, developed an index-type predictor to determine when A. porri will produce conidia in sufficiently large numbers to justify a fungicide application. This predictor uses hourly temperature and hourly relative humidity data to generate 3 daily factors described in Table 1.

Table 1. Daily Index Values for PRI Calculation

Factor
3
2
1
0

Temperature Index (TI)
2200 D-1 to 1000 D-1*
Min & Max Temps**
13< Temp <30
Min & Max Temps
7< Temp <30
Min & Max Temps
<7 or >30

Night RH Index (NRHI)
(hours >95%) 2200 D-1 to 1000 D-1
>8 Hr
4<Hr<7
<4 Hr

Low RH Index (LRHI)
2200 D-1 to 1200 D-0
<75% RH
76%<RH<85%
>85%

* D-0 = prediction date, D-1 = previous day, and D-2 = 2 days previous; military time.
** Degrees Celsius.

Values derived for the 3 factors are then used in the following formula to calculate the daily predictive index (PRI) value.

PRI = ((TID-2 + TID-1 + TID-0 + NRHID-2 + NRHID-1 + NRHID-0) / 3 + LRHID-0).

The Temperature Index (TI) and Night Relative Humidity Index (NRHI) predict spore formation by A. porri, and the Low Relative Humidity Index (NRHI) predicts spore release.