Jan Byrne
Diagnostic Services

Bacterial leaf spot of begonia is caused by Xanthomonas campestris pv. begoniae. Symptoms include water soaked lesions of the foliage, which are especially visible from the underside of the foliage. Severely affected leaves may fall off the plant. This bacterium can also become systemic, meaning it colonizes the vascular tissues of the plant, causing wilting symptoms.

When I mention Xanthomonas most growers automatically think of geraniums. The good news is that this is a different “kind” or pathovar of Xanthomonas. Bacterial leaf spot on begonia is caused by a Xanthomonas that can only infect plants in the genus Begonia. There are many different types of begonias, most of which can be affected, there are however differences in susceptibility among them. There are few specific cultivars available that are reported to be resistant.

The disease is frequently spread via contaminated plant material. Within the greenhouse, spread occurs when irrigation water splashes from an infected plant to the foliage of nearby hosts. Research has shown that the bacterium can be released from roots of systemically infected plants into ebb-and-flood irrigation systems. Theoretically, this allows for movement of the bacterium to healthy plants, but this is not typically a significant means of dissemination.

Disease management begins with healthy plant material. Inspect incoming begonias for signs of disease. Plants with any suspicious symptoms should be tested at a diagnostic lab. Please note that diagnostic test kits for X. campestris pv. pelargonii (bacterial wilt of geranium) cannot be used to detect X. campestris pv. begoniae. Diseased plant material should be removed and destroyed. Avoid handling plant material when the foliage is wet. Overhead irrigation should be carefully timed to minimize the amount of time that the foliage will remain wet. Applications of copper based fungicides can be used to reduce disease spread. Plants that are systemically infected can not be successfully treated.

Roberto Lopez and Matthew Blanchard
Horticulture

Most greenhouse growers use plant growth retardants (PGRs) to control the height of their spring and summer crops. However, PGRs have several disadvantages: they can be expensive to apply, can cause stunting if applied at an excessive rate, and their effect may persist in the landscape. There are several environmental height control strategies, such as light and temperature manipulation, that are discussed below. In the next Greenhouse Alert, cultivar selection and cultural strategies for height control will be discussed.

Light quality
Flowering and stem elongation is influenced by the color of light that plants receive, which is known as light quality. Sunlight has similar amounts of red (R) and far red (FR) light. Below a leaf canopy, the amount of FR light is about 20 times higher because leaves absorb R light and transmit FR. The FR light that is transmitted below a plant canopy or an individual leaf will cause shade-avoiding plants such as petunia and poinsettia to elongate rapidly. Therefore, when plants are tightly spaced, most plants respond by rapidly elongating or stretching and the lower leaves often turn yellow and senesce. Plants that naturally grow in shaded environments (such as hosta) do not usually exhibit the shade-avoidance response. Therefore, plants should be spaced as far apart as economically possible so that leaves from one plant do not touch the leaves of an adjacent plant. Also, don’t forget that hanging baskets can also induce the shade-avoidance response on the crop below.

Artificial light sources also influence the amount of red light and far red light that crops receive. Incandescent lamps emit more FR than R light, and thus promote stem elongation. On the other hand, high-pressure sodium (HPS) lamps have about six times more red light than far-red light. The effects of light quality can be seen in the accompanying figures, where Coreopsis ‘Early Sunrise’ grown under incandescent lamps (INC) were taller than plants grown under cool-white fluorescent (CWF), HPS, and metal halide (MH) lamps.

Table 1 (below)lists the ratios of red to far-red (R:FR) and other important factors for several lamps that are commonly used in greenhouses and germination chambers. Using HPS lamps to provide photoperiodic lighting can reduce plant stretch, however, the initial cost to purchase HPS cost is higher than for incandescent lamps.

DIF = day temperature – night temperature

The difference between the day and night temperature can influence internode elongation and thus stem extension. Stem elongation is promoted when the day temperature is warmer than night temperature (positive DIF). During the opposite environmental conditions, when day temperature is cooler than the night temperature (negative DIF), stem elongation is inhibited. A zero DIF refers to conditions where day and night temperatures are equal.

Knowledge of how DIF affects stem elongation can be used to regulate plant height. For example, plants grown at a day temperature of 68°F and a night temperature of 50°F (+18°F DIF) will have a taller finished height than plants grown at a day temperature of 68°F and a night temperature setpoint of 77°F (−9°F DIF) ( Figure 2). An intermediate height response would occur if these plants were grown at a day and night temperature of 68°F (0 DIF).

Although using a negative DIF may sound like a great environmental tool to control plant height, this technique may not be cost-effective with the current high cost of energy for greenhouse heating. However, by understanding the DIF concept you can avoid environmental conditions that promote unwanted plant stretch. For example, in response to increasing energy costs, many growers are lowering the night temperature setpoint to save on heat. When using this strategy to reduce your energy bill, remember that you are creating a positive DIF environment and stem extension will be promoted. In addition, you may be delivering a lower average daily temperature, and thus crop timing could be delayed.

Table 1. Comparisons among some common lamp types used in greenhouses. R = red light and FR = far-red light.

Dave Smitley
Entomology

Imidacloprid, flonicamid and some of the new neonicotinoids give excellent control of aphids when applied as a soil drench and used systemically.  But at this time we need sprayable products.  Two products that are especially good against aphids are Endeavor and Aria.  Other products that have worked well for aphids in the past include:  BotaniGard, Celero, Decathlon, Discus, Flagship, Marathon, Orthene 97, Precision, Talstar and Tristar.

Good scouting is critical to catch aphids early. They don't fly until populations reach a very high level, so if you see them on sticky cards, you probably have some hot spots that are loaded with aphids.