Virus Diseases of Petunia
Petunia x hybridia Hort.is one of many members of the family Solanaceae grown primarily as anornamental plant, and the most economically important ornamental member of thisfamily due to its horticultural value. It is a popular bedding plant the third most valuable after geranium and impatiens but is becomingmore popular as a colorful addition to container gardens, hanging baskets andwindow boxes. Most petunia varieties propagate by seed, but new species ofvegetatively propagated single petunias, such as ‘Supertunia’ and’Wave’ petunia have been introduced as garden ornamentals. Besidesthe traditional single petunia, the less-popular double petunia plants providesome of the most impressive flowers of all bedding plants. As vegetative propagation becomes more popular and the industry moves further away from seed-based propagation, the likelihood of viruses and virus-induced diseases in these crops becomes more of an issue.
The purpose of this article is to bring the reader up to speed on the subject of petunia viruses and the diseases they cause in this popular ornamental crop.
At my most recent count, there were approximately 130 plantviruses reported to infect petunias too many to be covered in thisarticle. They range from alfalfa mosaic virus to wineberry latent virus. Ofthese 130 viruses, there are a baker’s dozen that play a significant rolein causing economically important losses in petunia. They are: alfalfa mosaicvirus (AMV); arabis mosaic virus (ArMV); chrysanthemum stunt virus B (CVB);cucumber mosaic virus (CMV); impatiens necrotic spot virus (INSV); and selectmembers of the potyvirus group, which includes tobacco etch virus (TEV),tobacco mosaic virus (TMV), tobacco ringspot virus (TRSV), tobacco streak virus(TSV), tomato aspermy virus (TAV), tomato mosaic virus (ToMV), tomato ringspotvirus (ToRSV) and tomato spotted wilt virus (TSWV). These viruses cover a widevariety of virus families and vary greatly in their degree of disease-causingcapability. Within this group of 13, there are six viruses that cause diseasein petunias more frequently than any of the others. I will cover these indetail.
TMV. The virus mostoften detected in petunias in the United States is TMV. TMV is one of the mostcommon and most destructive plant viruses in the world. Its host range is verybroad, and it infects many species within the plant family Solanaceae, of whichpetunia, tomato, tobacco and peppers are some of the most important. TMV istransmitted from plant to plant primarily by mechanical means, e.g. pruning,handling of plants and plant-to-plant contact. One TMV-infected petunia in agreenhouse can be the source of infection for the entire crop, and it wouldonly take a matter of a few days for the entire crop to become infected. In myopinion, this is the worst virus to deal with in a petunia crop.
Symptoms associated with TMV infection are few and somewhatnondescript. The primary symptom is mosaic. TMV-associated mosaic can rangefrom severe to mild, depending on the petunia variety and the strain of TMV.TMV can also cause deformation and stunting of the leaves. Severe deformationmay result in the leaves becoming “rat-tailed.” Rat-tailed leavesare narrow and spindly. In some cases, TMV-infected plants can have flowersthat express color breaking. Color breaking of flowers is a common symptomassociated with virus infection and can vary in severity. Any one or all ofthese symptoms can be seen on plants infected with TMV.
INSV and TSWV. INSV and TSWV are closely related viruses thathave the potential to cause widespread damage in petunias. Like TMV, the hostrange of these two viruses is very broad and includes many commonly grownbedding and potted plants. Unlike TMV, INSV and TSWV are not as easilytransmitted from plant to plant by mechanical means. These two viruses movefrom plant to plant with the help of thrips, primarily Western Flower Thrips.Thrips are very efficient vectors of INSV and TSWV, and under conditions ofhigh thrips populations, one infected plant can serve as an infection sourcefor the entire greenhouse.
Symptoms associated with INSV and TSWV in a petunia canvary, depending on the petunia variety and at what stage of plant developmentthe initial infection took place. One of the most common symptoms is necroticringspots and/or necrotic flecking of the leaves. However, ringspots andnecrotic flecking cannot always guarantee the presence of INSV and TSWV sinceother petunia viruses may manifest symptoms in the same way.
ToRSV and TRSV. LikeINSV and TSWV, tomato ringspot virus and tobacco ringspot virus are closelyrelated and cause the infected plant to express similar symptoms. One of themost common symptoms is ringspots. ToRSV and TRSV can also cause severe leafdeformation and necrosis. If plants are infected at an early stage ofdevelopment, the end result may be death of the entire plant. Without the aidof a laboratory analysis, it is virtually impossible to distinguish ToRSV fromTRSV based solely on symptoms. These two viruses are not insect-vectored and donot present nearly the destructive potential that INSV and TWSV possess. Theyare not detected as commonly as the other viruses that we have discussed sofar, yet are worthy of mentioning due to the destructive nature of the symptomsthey manifest in the host plant.
CMV. Cucumber mosaic virus is the most common plant virus inthe world. Though it is rarely found in petunias, it is important to mentiondue to the extensive nature of this virus’ host range. Probably 75percent of the bedding plants, potted plants, herbaceous perennials andvegetables grown in today’s modern greenhouse are susceptible to CMV. Itis vectored by many common greenhouse aphids such as the green peach aphid andmelon aphid. In most cases, the virus induces a mild mosaic on leaves and colorbreaking on the flowers.
ToMV. This virus isa close relative of TMV and causes similar types of symptoms. The virus ismechanically transmitted like TMV, only not as easily. Like TMV, there are avariety of tomato mosaic virus strains, and symptoms can vary depending on thepetunia variety and the virus strain.
The Others. Theviruses that we have just covered are the most commonly found and/or have thepotential to be the most destructive. The other seven viruses can bedestructive, but they are usually found in low frequencies in individualgreenhouses where some unique situation has initiated infection. A recent,2-year study at The Ohio State University of 544 double petunia plants randomlyselected from greenhouses throughout Ohio indicated that TMV, ToRSV, TRSV, TSWVand TSV were the viruses responsible for virus-induced diseases in 1997 and1998 (See Table 1, page 25). The results of this 2-year study indicate that theviruses identified vary as to type and frequency from year to year. TSV (a rarevirus to find in a relatively small survey) was the only virus detected that wehave not previously discussed in this article.
The other interesting fact associated with this study is thefrequency of mixed infections. Plant viruses can frequently be found infectingplants in combination with other viruses. In this study, a new petunia viruswas identified that appeared to be closely related to TMV and ToMV; however,its DNA sequence indicated it was a distinct virus. This virus was designatedas petunia virus (PV). In 1998 PV was found in 13 samples mixed with TMV (seeTable 1, page 25). Based on the variation observed in this study, it is likelyif a study were conducted this year, results would vary from what we saw fouryears ago.
Control of petunia viruses
As with all plant viruses, the best method of control isprevention. Remember that there is Á no such thing as a systemicvirus-cide. Once a plant is infected, it is infected until the plant dies.Plants that are infected with any of these viruses should never be used as asource of material for vegetative propagation unless the individual plant hasbeen subjected to heat therapy and meristem tip culture. Even this can be riskyfor some of the more stable viruses such as TMV and ToMV. In all circumstances,only plants that have been verified by the producer to be”virus-free” should enter the greenhouse and only when thesevirus-free plants have been quarantined from the rest of the crop for a periodof time that would allow them to manifest virus symptoms and/or insectinfestations. Suspect plants should be removed from the greenhouse and testedfor a panel of common petunia viruses by a university or private lab. For INSVand TSWV, thrips control is essential. Using yellow or blue sticky cards tomonitor for thrips and other insects while plants are still in quarantine canhead-off disaster. Once plants have been moved out of the quarantine area,insect populations should be continually monitored.
Using seed to propagate petunias has always been a good wayto eliminate a virus problem in the greenhouse; however, seed propagation isbecoming less and less a method of petunia propagation with more and morevegetative propagation taking place. Also, viruses such as CMV, ToRSV and TRSVcan be seed-transmitted. Thus it is essential that the seed you purchase bedesignated by the producer as “virus-free.”
As more and more vegetative propagation takes place, andmore and more new varieties are entering the market each year, more and morevirus problems will become evident. There will be more viruses identified, andin some cases, novel viruses will be discovered. One such virus is the PV wediscovered in our research. Because of this, new and more sensitive methods ofvirus identification will need to be developed. These methods can be applied tovegetative- and seed-based propagation material to help ensure a virus-freepetunia crop.