Zucchini Blossom End Rot

I've been seeing some blossom end rot (BER) in my zucchini. The zucchini plants that I'm growing are volunteers from last year and are doing quite well, but some of the first fruit were showing signs of blossom end rot (see photos below). I always knew it had something to do with a calcium deficiency but wasn't quite sure about all of the particulars. 

As it turns out, calcium, or the lack of, plays a part in blossom end rot of zucchini as well as tomatoes, peppers and watermelon. I was always under the impression that the soil was deficient in calcium, but as it turns out, that's typically not the case. Most soils should have sufficient calcium to support health zucchini, etc. pH can, however, play a big factor in the availability of the calcium to the plants. In acidic soils, soils with a pH down around 5.5 or less, problems with blossom end rot are likely to persist throughout the season. The calcium is 'locked up' to the soil particles (clay particles and organic material particles) in low pH soils making the calcium unavailable for plant uptake. 

Adding a source of calcium to the soil isn't really a solution to correct a current problem. It actually takes several months to become available to plants. And the addition of calcium in already high pH soils can cause problems or deficiencies of other nutrients. Gypsum (calcium sulfate) will provide a source of calcium, but again it needs several months to become available to the plants. Sources of calcium are typically best added in the fall to be available for spring planting. Another possibility is calcium chloride often referred to as "Blossom End Rot Spray." It's water soluble and can be sprayed as a foliar spray over the entire plant. This is supposed to mitigate the problem of blossom end rot as well as being used to set flowers on zucchini, tomatoes, peppers and watermelon. 

Now I like to say that I live on a "sand pile." The soil here in this part of Orange County, California is a silty-sand with a pH very close to neutral or 7.0. And the blossom end rot on my zucchini comes and goes - not persistent. It should also be said that none of the problems causing blossom end rot are caused by disease causing organisms called pathogens. Blossom end rot is caused by various non-living factors like nutrient deficiencies, weather, watering habits and soil drainage. So I had to look elsewhere - referred to as an 'abiotic' disorder. 

As it turns out, there are some other common causes of blossom end rot on zucchini, tomatoes, peppers and watermelon. These problems are more likely to show up as sporadic problems and not continual like those in heavy, low pH soils. Some of the other problems include:

• Harsh changes in temperature (hot or cold snaps) when the fruit are setting
• Low nighttime temperatures
• Excessive irrigation or rainfall
• Heavy soils (clays) causing poor drainage 
• Excessive use of high-nitrogen (N) fertilizers

Now some of these are virtually impossible to control, but we do the best we can to sort it out and determine what the problem is. I'm pretty sure it isn't excess nitrogen fertilizers. Though I do use a commercial fertilizer, it is only 6% nitrogen. The overall temperatures have pretty mild so far this spring so that's probably not it. It has been a bit chilly the last few nights. I suppose that might be a cause. But I'm pretty sure that my problem was too much irrigation. I left the hose on a trickle around the plants and overly saturated the area just about the time the fruit were setting. I'm betting  - and hoping - that was the problem. I'll let you know. 

Some solutions to mitigate blossom end rot where you're pretty sure pH isn't the issue include:

• Addition of compost into heavy, clay soils to encourage better drainage
• Adding compost to sandy soils helps maintain a more balanced soil-moisture level
• Build raised planter beds
• Simply raise your rows higher 
• Install drip irrigation or ooze hose with an irrigation controller 
• When watering by hand - wait until morning to see if plants are still wilting and water then - plants often wilt in the afternoon and recover over night

On a side note:

One way I use to extend my growing season for zucchini as well as other squash and cucumbers, peppers, eggplant, bush beans and more is to build a hoop-house using PVC pipe and 6 mil painter's plastic. It works wonders when low nighttime temperatures are the issue. I'll put up plans in a future post. 

Thanks for lookin'!

Blossom end rot on zucchini

My volunteer plants

Healthy zucchini