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December 2018 / January 2019

Postharvest treatments aid control of Citrus Black Spot (CBS)

SA Fruit Journal: December 2018 / January 2019

Wouter Schreuder (a), Wilma Du Plooy (b), Arno Erasmus (b), Catherine Savage (b), Elaine Basson (b), Cheryl Lennox (a) And Paul H. Fourie (a,b)

a Department of Plant Pathology, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa
b Citrus Research International, P.O. Box 28, Nelspruit 1200, South Africa Citrus fruit with CBS lesions CBS lesion with pycnidia.

Citrus Black Spot is a fungal disease causing black lesions on fruit. Apart from this cosmetic defect, it has no other harmful effects on citrus fruit. However, due to the quarantine status of the disease in certain countries, CBS is a very important disease to control on citrus for fresh fruit exports.

Citrus Research International (CRI) conducts research with a wide range of collaborating research organisations. The work reported on here is from the MSc. study of Wouter Schreuder Jnr., whose thesis at Stellenbosch University was titled: Singular and combined effect of postharvest treatments on viability and reproductive ability of Phyllosticta citricarpa infections. The work studied the effect that postharvest packhouse treatments have on citrus black spot.

Why this project?

Citrus black spot (CBS) is caused by P. citricarpa and whereas preharvest fungicide treatments can provide high levels of control, EU phytosanitary regulations for the import of citrus fruit require the fruit to be completely CBS-free. Previous research has shown that postharvest treatments and cold conditions can make an important contribution to CBS control. However, postharvest treatment programmes have changed over time, and it was important to re-evaluate the effects of postharvest handling on CBS infections.

How was this project done?

Eureka lemons and Valencia oranges were harvested from infected orchards that had not received preharvest CBS sprays. Only fruit that showed no symptoms were harvested and brought back to the lab, because asymptomatic fruit will typically be exported. The fruit were then subjected to the various treatments and associated fungicides that are commonly used in commercial citrus packhouses in South Africa. These treatments were a chlorine wash; a pre-packhouse drench (pyrimethanil, thiabendazole, 2,4-D, and guazatine or propiconazole); a heated fungicide dip (imazalil at 25, 35, 45 and 55°C); and a wax application (thiabendazole, imazalil, and 2,4-D).
These treatments were applied individually, as well as in sequential combination as would happen in a citrus packhouse. Additional treatments included dip-treatments in suspensions of propiconazole, a mixture of pyrimethanil and propiconazole, fludioxonil, potassium sorbate, sodium bicarbonate, and Fortisol Ca Plus.

The effects of simulated shipping and cold storage conditions (five weeks at 7°C for lemons and 4°C or -0.5°C for Valencias) were evaluated, both on their own and in combination with the packhouse treatments.

After the five week storage period, the fruit were moved into an incubation room (25 – 27°C) with constant light to stimulate lesion development. Fruit were inspected after one and two weeks, and CBS lesions that developed were counted and recorded. Lesions that developed pycnidia (fruiting bodies that produce reproductive spores) were also recorded. Realtime PCR was used to positively confirm that the lesions were indeed CBS lesions.

Citrus fruit with CBS lesions

BS lesion with pycnidia.

What was found:

Statistical analysis of the results showed several interesting findings:

  • More than 93% of the observed lesions tested positive for genetic material of P. citricarpa (CBS).
  • On average, only 10 – 15% of untreated lesions developed fruiting bodies, and this low reproductive capability was in most cases further diminished by the combination of treatments followed by cold storage, with < 0.8% of new lesions forming pycnidia in most trials.
  • Shipping and storage under cooling (7°C, 4°C and -0.5°C) on their own resulted in low to moderate, but variable levels of control of CBS infections (23 – 69%).
  • Individual dip-treatments of additional products led to inconsistent levels of control.
  • The combination of postharvest treatments demonstrated consistent levels of control: Eureka lemons (average 65%) and Valencia oranges (average 76%).
  • Cold storage subsequent to packhouse treatments (as is common shipping procedure) further improved the levels of control to 81% on Eureka lemons and 94% on Valencia oranges.
  • Lesions covered in wax did not release spores, indicating that wax creates a physical barrier over the spores.


This study confirmed previous studies (Agostini et al., 2006; Korf et al., 2001; Rappussi et al., 2009, 2011; Seberry et al., 1967; Yan et al., 2016) that CBS lesions on fruit have very low reproductive ability and that high and consistent levels of CBS control can be achieved by standard packhouse treatments and cold storage. These findings, considered on their own and especially in combination with the epidemiological requirements for spore release and dispersal from fruit and subsequent infection (i.e. wetness, water dispersal over short distances, and long wetness periods on susceptible host tissue for infection) (Kiely, 1948; McOnie, 1965; Spósito et al., 2008, 2011), support the previous CBS pest risk assessments (South African CBS PRA, 2000-2009; USDA-APHIS, 2010) that fruit, especially fruit produced and harvested for commercial export, is not a likely pathway for the spread of CBS.


Agostini, J., Peres, N., Mackenzie, S., Adaskaveg, J., Timmer, L., 2006. Effect of fungicides and storage conditions on postharvest development of citrus black spot and survival of Guignardia citricarpa in fruit tissues. Plant Dis. 90, 1419-1424.

Kiely, T.B. 1948. Preliminary studies on Guignardia citricarpa (n. sp.), the ascigerous stage of Phoma citricarpa McAlp., and its relation to blackspot of citrus. Proc. Linn. Soc. N.S.W. 73, 249-292.

Korf, H.J.G., Schutte, G.C., Kotzé, J.M., 2001. Effects of packhouse procedures on the viability of Phyllosticta citricarpa, anamorph of citrus black spot pathogen. Afr. Plant Prot. 7, 103-106.

McOnie, K.C., 1965. Source of infection for black spot of citrus. S. Afr. Citrus J. 378, 5-6, 9.

Rappussi, M.C.C., Pascholati, S.F., Benato, E.A., Cia, P., 2009. Chitosan reduces infection by Guignardia citricarpa in postharvest ‘‘Valencia’’ oranges. Brazilian Arch Biol Technol 52:513–521

Seberry, J.A., Leggo, D., Kiely, T.B., 1967. Effect of skin coatings on the development of black spot in stored Valencia Oranges. Aust. J. Exp. Agric. Anim. Husb. 7, 593-600.

South African CBS PRA, 2000-2009. Citrus Black Spot Consolidated Pest Risk Assessment Pertaining to the Export of Fresh Citrus Fruit from the Republic of South Africa to the European Union.

Spósito, M.B., Amorim, L., Bassanezi, R.B., Bergamin, F.A., Hau, B., 2008. Spatial pattern of black spot incidence within citrus trees related to disease severity and pathogen dispersal. Plant Pathol. 57, 103-108.

Spósito, M.B., Amorim, L., Bassanezi R. B., Yamamoto P.T., Felippe, M.R., Czermainski, A.B.C., 2011. Relative importance of inoculum sources of Guignardia citricarpa on the citrus black spot epidemic in Brazil. Crop Prot. 30, 1546-1552.

USDA-APHIS, 2010. Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes citrus black spot disease. Center for Plant Health Science and Technology, Plant Epidemiology and Risk Analysis Laboratory, Raleigh, NC, USA.

Yan, J., Dewdney, M.M., Roberts, P.D., Ritenour, M.A., 2016. The effects of postharvest hot water and fungicide treatments on Guignardia citricarpa growth and the development of citrus black spot symptoms on ‘Valencia’ orange fruit. Hortsci. 51, 1555–1560.

Read the full published article: Schreuder, W., du Plooy, W., Erasmus, A., Savage, C., Basson, E., Lennox C. and, Fourie, P.H., 2018. Postharvest fungicide treatments and cold storage control citrus black spot infections. Crop Protection, 112, 332-342.

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