MAY, 2010      

VOL. 24, NO. 5                                                                             




Citrus Black Spot

By now, probably everyone has heard of the discovery of citrus black spot disease in Collier County, Florida.  Citrus black spot is a fungal disease caused by Guignardia citricarpa (Kiely) that results is significant fungal spots on the fruit and foliage of citrus.  The primary loss is in fresh fruit, as the blemishes eliminate the fruit from fresh market use.  However, the disease can also result in premature and significant fruit drop, thus lowering productivity.

A related organism, G. mangiferae, is apparently widespread on guava, avocado, mango and passion fruit in Florida and has been isolated from citrus fruit spots, the latter causing some confusion with citrus black spot disease.  However, further testing (and better tests) can and do distinguish between the two organisms.

Most citrus cultivars are susceptible to some degree, but lemons, limes, mandarins and grapefruit are especially vulnerable, while late oranges (Valencia) can suffer significant yield losses.  Both Tahiti lime and sour orange are considered not to be susceptible.

The disease is most common in subtropical climates with rainy summers.  It is known in Australia and several countries in Africa, Asia, and South America (Brazil, Argentina, and Uruguay).  Florida is a subtropical area with rainy summers, so the disease should flourish there.

In the Florida discovery, initial detection resulted from a CHRP (Citrus Health Response Program) survey requested by the grower of a commercial Valencia orange orchard, the fruit of which was grown for processing.  Florida Department of Agriculture and Consumer Services, Division of Plant Industry, made the initial identification on March 25, after which DPI and APHIS-PPQ personnel returned to the grove to collect samples for confirmation in Beltsville, and the identification was confirmed on April 7.

How did it get there?  Initial reports are that while the symptoms are clearest on the fruit, spread via fruit movement is considered minimal, with the greatest means of spread via sporulation from fallen, decomposing leaves (like greasy spot).  But that doesn’t answer the question, as one is then left to wonder how there came to be fallen, decomposing leaves with the disease.  Too, an IFAS paper written a couple of years ago suggests that the disease could be introduced via the movement of infected fruit or the illegal introduction of vegetative materials (leaves). 

In addition, a 2008 publication from Brazil [Plant Pathology 57(1):103-108] indicates that splash-dispersed conidia (asexual spores) on the fruit play an important role in increasing the disease incidence in trees.  In other words, once the lesions appear on the fruit et cetera, the asexual spores can result in additional infections within the tree.  Still, the main spread is from sexual spores released from decaying leaves.

Finally, the fruit is apparently susceptible to infection for four or five months after petal fall—i.e., until July or August.  However, the organism has a long latency period which may preclude the development of fruit symptoms until the fruit has matured.  This suggests that the disease organism must have been introduced prior to the end of the susceptibility period last summer.

You may recall reports of the interceptions of mandarins (infected with citrus black spot) in North Dakota last fall and winter.  In those cases, Customs and Border Protection discovered infected mandarin fruit in the vehicles of Canadian travelers bound for vacations in the Sunbelt.  Yet, given the information on infection and manifestation of symptoms, the disease had to have been introduced into Florida months before these interceptions. 

One could speculate that infected fruit coming in with travelers from Canada may have escaped CBP scrutiny in the fall and winter of 2008-09, thereby leading to the disease in 2009-10.  Whether or not that is valid, we should probably be more vigilant regarding the possibility that citrus black spot might be present locally—or might appear in the next 12 months.  While we don’t exactly have “rainy summers”, we do irrigate, which seems likely to facilitate spore release in G. citricarpa just as it does for Mycosphaerella horii (greasy spot).

Periwinkles and Citrus Greening

Scientists with the USDA-ARS and the Indian River Research and Extension Center report that the common garden periwinkle is a good substitute for studying citrus greening.  Greenhouse-grown periwinkles were infected with the citrus greening bacterium from infected lemon trees by using dodder to effect the transfer.  They then used the periwinkles to test optimal nutrient and soil treatments to regenerate periwinkle having high infection rates.   They also discovered a couple of chemicals that provided some success in treating the infected periwinkle.  The names of the chemicals are not so important at this time, as much testing is yet to be conducted.

Uncommon Weather?

April rains bring May flowers, or so the lyric goes, but our wildflowers come before May and we don’t often get rain in April.  To the chagrin of the sugar mill and the dismay of onion growers, mid-April rains fell all across the Valley and South Texas, with the greatest amounts in the Upper Valley—some growers reported more than 5.0 inches.  In the Mid-Valley and Lower Valley, totals were less than half that, with reports of 2.0-2.5 inches being common.

Temperatures have started the seasonal climb towards triple digits, with the end of April recording highs in the 90s. 

Citrus Movement

The overall fruit volume has been slowing since about mid-March, but the season is still going.  According to TVCC calculations, about 13 percent of the estimated grapefruit crop was yet to be picked as of mid-April, with about 16 percent of the estimated Valencia crop still on-tree. 

Fresh movement is ahead of last season’s year-to-date volume, and is only a couple of days shipping behind last season’s final volume.  In reality, because of the lag time for TVCC reports, there is little question that this season’s volume has already surpassed last season’s total.

Some growers may be unaware that there has been no change in the volume of oranges diverted to processing for several weeks.  There may be several underlying reasons, but the essence of the matter is that the total volume of oranges available from the packinghouses to the juice plant is insufficient for TCX to run, so no oranges are going to the juice plant.  In case you are wondering, a minimum of 75 tons is required for half a run. 

Will the season end in May?  Good question, but I don’t have the answer.  I think everybody would like to be done already, but extension into June would not be a big surprise.  There does appear to be quite a lot of grapefruit out there yet, but overall external quality and pack out percentages are slowly declining in a typical seasonal pattern.

Citrus Greening in Colima

The Servicio Nacional de Sanidad, Inocuidad y Calidad Agroalimentaria (SENASICA) reported on April 16 that one Asian citrus psyllid and five thorny Mexican lime trees collected in Tecoman, Colima, tested positive for the citrus greening bacterium.  That brings the total of states in Mexico that are quarantined for citrus greening to six—three in the Yucatan Peninsula and now three on the SW coast.


Professor & Extension Horticulturist
2401 East Highway 83
Weslaco TX 78596


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