Use Disease Resistant Vegetable Varieties in Your IPM Program

There has been a lot of disease pressure in Pennsylvania vegetable fields this year.  Late blight arrived early with the first report coming from tomatoes in Blair County on June 4th.  It continued to show up throughout the season, with confirmed reports in at least 21 counties so far.  Blossom end rot on tomato was severe in many parts of the state, following an extreme heat wave in June and into July.  Downy mildew was reported in cucumber fields in Lancaster County as early as June 22nd, and was found on many different types of cucurbits as the season progressed. 

One of the best management tools in an integrated pest management program is using resistant varieties.  It is inexpensive compared to the cost of pesticides, and the resistance is constantly present from the first day of the growing season.  As you choose varieties to grow next year, consider including disease resistant selections to help reduce loss from diseases on your farm. 

Remember, resistance in vegetable varieties does not mean they are completely immune to those specific diseases.  It means a specific variety has greater tolerance to a particular disease.  Disease resistant varieties may still be affected by the disease, but they typically have less damage than a non-resistant variety and often require fewer to no protectant fungicide sprays during the season.   

Disease resistant varieties are a great tool, but they are not infallible.  The pathogens that cause plant disease can change as they reproduce and can overcome resistance in the plant.  Disease resistant varieties also cannot overcome other problems like inadequate fertility, irrigation, or insect, mite or weed pressure.  Growers need to constantly watch what is happening in their fields and be prepared to respond to problems that develop. 

Late summer is a great time to look at disease resistant varieties, to see how they have reacted to disease pressure, and how productive they were.  Are your neighbors growing something you would like to try next year?  Ask around and try to make time to go take a look at it.  Also, you might find a variety you are interested in for sale at a farm stand and have a chance to taste it before deciding to grow it next year. 

Where can you find specific information on disease resistance in vegetable crops?

Try Dr. Margaret McGrath's website at http://vegetablemdonline.ppath.cornell.edu.  Click the resistant varieties tab on the left to find an impressive amount of information about disease resistance listed in tables by crop.  If you need a hard copy of the tables, contact your county extension office. 

There has been a lot of work to develop late blight resistant varieties of tomatoes.  Dr. Randy Gardner from North Caroline State University has released several varieties with two genes that confer resistance to late blight.  They are 'Mountain Merit' which is a large-fruited determinate type and 'Mountain Magic' which is a smaller-fruited indeterminate type.  Another new variety with multiple genes for resistance is 'Defiant', a mid-size slicing, determinate tomato.  Varieties that have one gene for resistance to late blight include 'Plum Regal', a plum type, and 'Legend', a larger-fruited determinate type.  New varieties with new combinations of genetic resistance are being developed all the time.  In general, look for late blight resistant varieties that have multiple genes for resistance, and still have the other characteristics you are looking for like taste, color, fruit size, plant type and days to maturity.  Varieties with multiple resistance genes should hold up against late blight better than varieties with single resistance genes.   

The tomato varieties listed above may have some resistance to additional diseases as well.  For example, 'Defiant' also has resistance to early blight, Fusarium wilt (Races 1 and 2) and Verticillum wilt.  Check the descriptions in the seed catalogs to see the specific disease resistance reported for each variety. Also be aware of what diseases are commonly found in your region.

Dr. McGrath lists 18 tomato varieties with reported resistance to blossom end rot, including 'Mountain Spring', 'Burpee Supersteak' and 'German Pink'.  If you suffered damage from blossom end rot this year, first consider improving your ability to uniformly irrigate your crop, which is the best way to reduce this problem.  Beyond that, planting these varieties may give you some insurance against blossom end rot.

Here's a list of cucumber varieties recommended for use in Pennsylvania, which also have reported resistance to downy mildew and powdery mildew:  'Burpless 26', 'Diva', 'Dasher II', 'Fanfare', 'Indy', 'Marketmore 97', 'Orient Express', 'Sweet Slice', 'Tasty Green' and 'Tasty Jade'.  The resistance to these diseases is no longer very strong in cucumbers because the pathogens have changed, but these varieties may do better than those with no reported resistance. 

Disease resistant vegetable varieties are constantly being developed and introduced.  Disease causing fungi and other pathogens are constantly changing, and can overcome resistance that held up in the past.  Some varieties not listed in this article may perform as well or better than those mentioned.  Keep watching those seed catalogs to learn of new options!

Emelie Swackhamer
Horticulture Educator, Penn State Extension, Lehigh and Northampton Counties