Dry Weather and Fungicide Applications – Vegetable Crops Hotline

Dry Weather and Fungicide Applications

As I write this, scattered showers are moving through much of southern Indiana. It is unclear yet how much moisture these showers will produce for any given area. What is clear is that May and June have been unusually dry for most of Indiana. How will this dry spell affect overall disease pressure for the 2023 season?

First, most foliar diseases require leaf wetness in order for the disease to occur. Some exceptions are listed later. For this reason, I recommend a fungicide spray schedule of 7-14 days for most purposes. More frequent fungicide applications should occur during wet weather when conditions are most conducive to foliar diseases. When the weather is dry, applications can be spread out to 14 days or so. Cantaloupe and watermelon growers have the guesswork taken out of fungicide scheduling by the Purdue program MELCAST.

If one assumes that most vegetable plantings were conducted in early May, much of Indiana vegetable production experienced dry weather for about 30 days. I predict that this will result in less overall disease for the 2023 season. While many growers will have to deal with the consequences of this abnormally dry period, it may be reassuring to know that foliar disease will probably not be as severe as in most seasons.

You may be thinking that the weather may change to rainy and change our fortunes. True. But the dry weather in the initial portion of the season can’t be changed and will probably lead to lower foliar disease levels overall. Let me explain.

Foliar disease severity is measured not only by the number of lesions that occur on foliage at the beginning of the season. But also by how many spores are produced by these lesions and whether the spores go on to produce additional lesions. That is, foliar disease severity is measured by the overall spread of the disease as the season progresses. Since the beginning of the season has been dry, little disease was initiated and little disease spread has occurred. So, even if the weather turns to a normal year weather-wise, it is very probable that overall, growers will observe less foliar disease on their crops.

Maybe an analogy is in order. Foliar disease increase can be compared to an interest accruing bank account. Let’s assume each rain equals a specific amount of foliar disease and is represented by a deposit in the account.  Foliar disease increases in relation to the length of leaf wetness, not the amount of rain. So, longer times of leaf wetness represent larger deposits. Let’s also assume that our bank account will have a definite time period, like a crop season.

Bank accounts earn interest in a similar way to the increase in the rate of foliar diseases with time. Therefore, rainy periods represent deposits in our account. The rate of disease spread represents interest in the account. Dry weather is similar to a period with no deposits in our bank account.

What we have witnessed in the first 30 days or so of the season is a lack of deposits of any significance. So, even if it starts to rain soon and deposits are made into our account, and interest begins to accrue, the amount of funds in our account will be less than in a normal year due to the lack of rain (or deposits) at the start of the season.

It is possible that the season will turn very rainy and we will end up with very serious foliar disease of our vegetables. However, this would mean that the remainder of the season is much more rainy than normal to counter our dry season beginning. If normal rains begin, I predict vegetable diseases for most growers and locations for 2023 will be less than the average year. I should mention that if one regularly uses overhead irrigation, this factor will increase the possibility of foliar diseases and negate much of this discussion.

Some foliar diseases do not require much in the way of leaf wetness and therefore are an exception to the above discussion. For example, powdery mildew of many crops requires only high humidity. Downy mildew of cucurbit crops, if it blows into Indiana this year, requires only heavy dews (last year, I observed no downy mildew of cucurbits).

Soil-borne fungi, which may cause root diseases or vascular wilts, are not affected by leaf wetness or rains in the same way as foliar diseases.

Note I am not predicting the weather. I am predicting overall vegetable disease severity for 2023.

2023 may be a year when you can end up with lower expenses for fungicides for foliar diseases. When it is dry, it may be possible to spread out foliar fungicide applications to about 14 days. It may also be possible to use less expensive contact products in contrast with expensive systemic products. However, all growers should maintain a 7-14 day schedule of fungicides for most situations. It is also important to continue to scout fields for diseases, insects and weeds.

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